CHAPTER XV

PHYSIOGRAPHIC AND GEOLOGIC DEVELOPMENT

GENERAL FEATURES

In the preceding chapter we employed geologic facts in the determination of the age of the principal topographic forms. These facts require further discussion in connection with their closest physiographic allies if we wish to show how the topography of today originated. There are many topographic details that have a fundamental relation to structure; indeed, without a somewhat detailed knowledge of geology only the broader and more general features of the landscape can be interpreted. In this chapter we shall therefore refer not to the scenic features as in a purely topographic description, but to the rock structure and the fossils. A complete and technical geologic discussion is not desirable, first, because it should be based upon much more detailed geologic field work, and second because after all our main purpose is not to discuss the geologic features _per se_, but the physiographic background which the geologic facts afford. I make this preliminary observation partly to indicate the point of view and partly to emphasize the necessity, in a broad, geographic study, for the reconstruction of the landscapes of the past.

The two dominating ranges of the Peruvian Andes, called the Maritime Cordillera and the Cordillera Vilcapampa, are composed of igneous rock--the one volcanic lava, the other intrusive granite. The chief rock belts of the Andes of southern Peru are shown in Fig. 157. The Maritime Cordillera is bordered on the west by Tertiary strata that rest unconformably upon Palaeozoic quartzites. It is bordered on the east by Cretaceous limestones that grade downward into sandstones, shales, and basal conglomerates. At some places the Cretaceous deposits rest upon old schists, at others upon Carboniferous limestones and related strata, upon small granite intrusives and upon old and greatly altered volcanic rock.

The Cordillera Vilcapampa has an axis of granitic rock which was thrust upward through schists that now border it on the west and slates that now border it on the east. The slate series forms a broad belt which terminates near the eastern border of the Andes, where the mountains break down abruptly to the river plains of the Amazon Basin. The immediate border on the east is formed of vertical Carboniferous limestones. The narrow foothill belt is composed of Tertiary sandstones that grade into loose sands and conglomerates. The inclined Tertiary strata were leveled by erosion and in part overlain by coarse and now dissected river gravels, probably of Pleistocene age. Well east of the main border are low ranges that have never been described. They could not be reached by the present expedition on account of lack of time. On the extreme western border of that portion of the Peruvian Andes herein described, there is a second distinct border chain, the Coast Range. It is composed of granite and once had considerable relief, but erosion has reduced its former bold forms to gentle slopes and graded profiles.

The continued and extreme growth of the Andes in later geologic periods has greatly favored structural and physiographic studies. Successive uplifts have raised earlier deposits once buried on the mountain flanks and erosion has opened canyons on whose walls and floors are the clearly exposed records of the past. In addition there have been igneous intrusions of great extent that have thrust aside and upturned the invaded strata exposing still further the internal structures of the mountains. From sections thus revealed it is possible to outline the chief events in the history of the Peruvian Andes, though the outline is still necessarily broad and general because based on rapid reconnaissance. However, it shows clearly that the landscape of the present represents but a temporary stage in the evolution of a great mountain belt. At the dawn of geologic history there were chains of mountains where the Andes now stand. They were swept away and even their roots deeply submerged under invading seas. Repeated uplifts of the earth’s crust reformed the ancient chains or created new ones out of the rock waste derived from them. Each new set of forms, therefore, exhibits some features transmitted from the past. Indeed, the landscape of today is like the human race--inheriting much of its character from past generations. For this reason the philosophical study of topographic forms requires at least a broad knowledge of related geologic structures.

SCHISTS AND SILURIAN SLATES[50]

The oldest series of rocks along the seventy-third meridian of Peru extends eastward from the Vilcapampa batholith nearly to the border of the Cordillera, Fig. 157. It consists of (1) a great mass of slates and shales with remarkable uniformity of composition and structure over great areas, and (2) older schists and siliceous members in restricted belts. They are everywhere thoroughly jointed; near the batholith they are also mineralized and altered from their original condition; in a few places they have been intruded with dikes and other form of igneous rock.

The slates and shales underlie known Carboniferous strata on their eastern border and appear to be a physical continuation of the fossiliferous slates of Bolivia; hence they are provisionally referred to the Silurian, though they may possibly be Devonian. Certainly the known Devonian exceeds in extent the known Silurian in the Central Andes but its lithological character is generally quite unlike the character of the slates here referred to the Silurian. The schists are of great but unknown age. They are unconformably overlain by known Carboniferous at Puquiura in the Vilcapampa Valley (Fig. 158), and near Chuquibambilla on the opposite side of the Cordillera Vilcapampa. The deeply weathered fissile mica schists east of Pasaje (see Appendix C for all locations) are also unconformably overlain by conglomerate and sandstone of Carboniferous age. While the schists vary considerably in lithological appearance and also in structure, they are everywhere the lowest rocks in the series and may with confidence be referred to the early Palaeozoic, while some of them may date from the Proteriozoic.

The Silurian beds are composed of shale, sandstone, shaly sandstone, limestone, and slate with some slaty schist, among which the shales are predominent and the limestones least important. Near their contact with the granite the slate series is composed of alternating beds of sandstone and shale arranged in beds from one to three feet thick. At Santa Ana they become more fissile and slaty in character and in several places are quarried and used for roofing. At Rosalina they consist of almost uniform beds of shale so soft and so minutely and thoroughly jointed as to weather easily. Under prolonged erosion they have, therefore, given rise to a well-rounded and soft-featured landscape. Farther down the Urubamba Valley they again take on the character of alternating beds of sandstone and shale from a few feet to fifteen and more feet thick. In places the metamorphism of the series has been carried further--the shales have become slates and the sandstones have been altered to extremely resistant quartzites. The result is again clearly shown in the topography of the valley wall which becomes bold, inclosing the river in narrow “pongos” or canyons filled with huge bowlders and dangerous rapids. The hills become mountains, ledges appear, and even the heavy forest cover fails to smooth out the natural ruggedness of the landscape.

It is only upon their eastern border that the Silurian series includes calcareous beds, and all of these lie within a few thousand yards of the contact with the Carboniferous limestones and shales. At first they are thin paper-like layers; nearer the top they are a few inches wide and finally attain a thickness of ten or twelve feet. The available limestone outcrops were rigorously examined for fossils but none were found, although they are lavishly distributed throughout the younger Carboniferous beds just above them. It is also remarkable that though the Silurian age of these beds is reasonably inferred they are not separated from the Carboniferous by an unconformity, at least we could find none in this locality. The later beds disconformably overlie the earlier beds, although the sharp differences in lithology and fossils make it easy to locate the line of separation. The limestone beds of the Silurian series are extremely compact and unfossiliferous. At least in this region those of Carboniferous age are friable and the fossils varied and abundant. The Silurian beds are everywhere strongly inclined and throughout the eastern half or third of their outcrop in the Urubamba Valley they are nearly vertical.

In view of the enormous thickness of the repeated layers of shale and sandstone this series is of great interest. Added importance attaches to their occurrence in a long belt from the eastern edge of the Bolivian highlands northward through Peru and possibly farther. From the fact that their disturbance has been on broad lines over wide areas with extreme metamorphism, they are to be separated from the older mica-schists and the crumpled chlorite schists of Puquiura and Pasaje. Further reasons for this distinction lie in their lithologic difference and, to a more important degree, in the strong unconformity between the Carboniferous and the schists in contrast to the disconformable relations shown between the Carboniferous and Silurian fifty miles away at Pongo de Mainique. The mashing and crumpling that the schists have experienced at Puquiura is so intense, that were they a part of the Silurian series the latter should exhibit at least a slight unconformity in relation to the Carboniferous limestones deposited upon them.

If our interpretation of the relation of the schists to the slates and shales be correct, we should have a mountain-making period introduced in pre-Silurian time, affecting the accumulated sediments and bringing about their metamorphism and crumpling on a large scale. From the mountains and uplands thus created on the schists, sediments were washed into adjacent waters and accumulated as even-bedded and extensive sheets of sands and muds (the present slates, shales, quartzites, etc.). Nowhere do the sediments of the slate series show a conglomeratic phase; they are remarkably well-sorted and consist of material disposed with great regularity. Though they are coarsest at the bottom the lower beds do not show cross-bedding, ripple marking, or other signs of shallow-water conditions. Toward the upper part of the series these features, especially the ripple-marking, make their appearance. During the deposition of the last third of the series, and again just before the deposition of the limestone, the beds took on a predominantly arenaceous character associated with ripple marks and cross-bedding characteristic of shallow-water deposits.

In the persistence of arenaceous sediments throughout the series and the distribution of the ripple marks through the upper third of the beds, we have a clear indication that the degree of shallowness was sufficient to bring the bottom on which the sediments accumulated into the zone of current action and possibly wave action. It is also worth considering whether the currents involved were not of similar origin to those now a part of the great counter-clockwise movements in the southern seas. If so, their action would be peculiarly effective in the wide distribution of the sediment derived from a land mass on the eastern edge of a continental coast, since they would spread out the material to a greater and greater degree as they flowed into more southerly latitudes. Among geologic agents a broad ocean current of relatively uniform flow would produce the most uniform effects throughout a geologic period, in which many thousand feet of clastic sediments were being accumulated. A powerful ocean current would also work on flats (in contrast to the gradient required by near-shore processes), and at the same time be of such deep and steady flow as to result in neither ripple marks nor cross-bedding.

The increasing volume of shallow-water sediments of uniform character near the end of the Silurian, indicates great crustal stability at a level which brought about neither a marked gain nor loss of material to the region. At any rate we have here no Devonian sediments, a characteristic shared by almost all the great sedimentary formations of Peru. At the beginning of the Carboniferous the water deepened, and great heavy-bedded limestones appear with only thin shale partings through a vertical distance of several hundreds of feet. The enormous volume of Silurian sediments indicates the deep and prolonged erosion of the land masses then existing, a conclusion further supported (1) by the extensive development of the Silurian throughout Bolivia as well as Peru, (2) by the entire absence of coarse material whether at the top or bottom of the section, and (3) by the very limited extent of older rock now exposed even after repeated and irregular uplift and deep dissection. Indeed, from the latter very striking fact, it may be reasonably argued that in a general way the relief of the country was reduced to sea level at the close of the Silurian. Over the perfected grades of that time there would then be afforded an opportunity for the effective transportation of waste to the extreme limits of the land.

Further evidence of the great reduction of surface during the Silurian and Devonian is supplied by the extensive development of the Carboniferous strata. Their outcrops are now scattered across the higher portions of the Andean Cordillera and are prevailingly calcareous in their upper portions. Upon the eastern border of the Silurian they indicate marine conditions from the opening of the period, but at Pasaje in the Apurimac Valley they are marked by heavy beds of basal conglomerate and sandstone, and an abundance of ripple marking and other features associated with shallow-water and possibly near-shore conditions.

CARBONIFEROUS

Carboniferous strata are distributed along the seventy-third meridian and rival in extent the volcanic material that forms the western border of the Andes. They range in character from basal conglomerates, sandstones, and shales of limited development, to enormous beds of extremely resistant blue limestone, in general well supplied with fossils. On the eastern border of the Andes they are abruptly terminated by a great fault, the continuation northward of the marginal fault recognized in eastern Bolivia by Minchin[51] and farther north by the writer.[52] Coarse red sandstones with conglomeratic phase abut sharply and with moderate inclination against almost vertical sandstones and limestones of Carboniferous age. The break between the vertical limestones and the gently inclined sandstones is marked by a prominent scarp nearly four thousand feet high (Fig. 159), and the limestone itself forms a high ridge through which the Urubamba has cut a narrow gateway, the celebrated Pongo de Mainique.

At Pasaje, on the western side of the Apurimac, the Carboniferous again appears resting upon the old schists described on p. 236. It is steeply upturned, in places vertical, is highly conglomeratic, and in a belt a half-mile wide it forms true badlands topography. It is succeeded by evenly bedded sandstones of fine and coarse composition in alternate beds, then follow shales and sandstones and finally the enormous beds of limestone that characterize the series. The structure is on the whole relatively simple in this region, the character and attitude of the beds indicating their accumulation in a nearly horizontal position. Since the basal conglomerate contains only pebbles and stones derived from the subjacent schists and does not contain granites like those in the Cordillera Vilcapampa batholith on the east it is concluded that the batholithic invasion was accompanied by the compression and tilting of the Carboniferous beds and that the batholith itself is post-Carboniferous. From the ridge summits above Huascatay and in the deep valleys thereabouts the Carboniferous strata may be seen to extend far toward the west, and also to have great extent north and south. Because of their dissected, bare, and, therefore, well-exposed condition they present exceptional opportunities for the study of Carboniferous geology in central Peru.

Carboniferous strata again appear at Puquiura, Vilcapampa, and Pampaconas. They are sharply upturned against the Vilcapampa batholith and associated volcanic material, chiefly basalt, porphyry, and various tuffs and related breccias. The Carboniferous beds are here more arenaceous, consisting chiefly of alternating beds of sandstone and shale. The lowermost beds, as at Pongo de Mainique, are dominantly marine, fossiliferous limestone beds having a thickness estimated to be over two miles.

From Huascatay westward and southward the Carboniferous is in part displaced by secondary batholiths of granite, in part cut off or crowded aside by igneous intrusions of later date, and in still larger part buried under great masses of Tertiary volcanic material. Nevertheless, it remains the dominating rock type over the whole stretch of country from Huascatay to Huancarama. In the northwestern part of the Abancay sheet its effect on the landscape may be observed in the knife-like ridge extending from west to east just above Huambo. Above Chuquibambilla it again outcrops, resting upon a thick resistant quartzite of unknown age, Fig. 162. It is strongly developed about Huadquirca and Antabamba and, still associated with a quartzite floor, it finally disappears under the lavas of the great volcanic field on the western border of the Andes. Figs. 141 and 142 show its relation to the invading granite batholiths and Fig. 162 shows further structural features as developed about Antabamba where the great volcanic field of the Maritime Cordillera begins.

Both the enormous thickness of the Carboniferous limestone series and the absence of clastic members over great areas in the upper portion of the series prove the widespread extent of the Carboniferous seas and their former occurrence in large interlimestone tracts from which they have since been eroded. At Puquiura they extend far over the schist, in fact almost completely conceal it; at Pasaje they formerly covered the mica-schists extensively, their erosion in both cases being conditioned by the pronounced uplift and marginal deformation which accompanied the development of the Vilcapampa batholith.

The degree of deformation of the Carboniferous sediments varies between simple uplift through moderate folding and complex disturbances resulting in nearly vertical attitudes. The simplest structures are represented at Pasaje, where the uplift of the intruded schists, marginal to the Vilcapampa batholith, has produced an enormous monoclinal fold exposing the entire section from basal conglomerates and sandstones to the thickest limestone. Above Chuquibambilla the limestones have been uplifted and very gently folded by the invasion of granite associated with the main batholith and several satellitic batholiths of limited extent. A higher degree of complexity is shown at Pampaconas (Fig. 141), where the main monoclinal fold is traversed almost at right angles by secondary folds of great amplitude. The limestones are there carried to the limit of the winter snows almost at the summit of the Cordillera. The crest of each secondary anticline rises to form a group of conspicuous peaks and tabular ridges. Higher in the section, as at Puquiura, the sandstones are thrown into a series of huge anticlines and synclines, apparently by the marginal compression brought about at the time of the intrusion of the granite core of the range. At Pongo de Mainique the whole of the visible Carboniferous is practically vertical, and is cut off by a great fault marking the abrupt eastern border of the Cordillera.

It is noteworthy that the farther east the Carboniferous extends the more dominantly marine it becomes, though marine beds of great thickness constitute a large part of the series in whatever location. From Huascatay westward the limestones become more and more argillaceous, and finally give way altogether to an enormous thickness of shales, sandstones, and thin conglomerates. These were observed to extend with strong inclination westward out of the region studied and into and under the volcanoes crowning the western border of the Cordillera. Along the line of traverse opportunity was not afforded for further study of this aspect of the series, since our route led generally along the strike rather than along the dip of the beds. It is interesting to note, however, that these observations as to the increasing amounts of clastic material in a westward direction were afterwards confirmed by Señor José Bravo, the Director of the Bureau of Mines at Lima, who had found Carboniferous land plants in shales at Pacasmayo, the only fossils of their kind found in Peru. Formerly it had been supposed that non-marine Carboniferous was not represented in Peru. From the varied nature of the flora, the great thickness of the shales in which the specimens were collected, and the fact that the dominantly marine Carboniferous elsewhere in Peru is of great extent, it is concluded that the land upon which the plants grew had a considerable area and probably extended far west of the present coast line. Since its emergence it has passed through several orogenic movements. These have resulted in the uplift of the marine portion of the Carboniferous, while the terrestrial deposits seem to have all but disappeared in the down-sunken blocks of the ocean floor, west of the great fault developed along the margin of the Cordillera. The following figures are graphic representations of this hypothesis.

The wide distribution of the Carboniferous sediments and especially the limestones, together with the uniformity of the fossil faunas, makes it certain that the sea extended entirely across the region now occupied by the Andes. However, from the relation of the Carboniferous to the basal schists, and the most conservative extension of the known Carboniferous, it may be inferred that the Carboniferous sea did not completely cover the entire area but was broken here and there by island masses in the form of an elongated archipelago. The presence of land plants in the Carboniferous of Pisco warrants the conclusion that a second island mass, possibly an island chain parallel to the first, extended along and west of the present shore.

CRETACEOUS

The Cretaceous formations are of very limited extent in the belt of country under consideration, in spite of their generally wide distribution in Peru. They are exposed distinctly only on the western border of the Cordillera and in special relations. In the gorge of Cotahuasi, over seven thousand feet deep, about two thousand feet of Cretaceous limestones are exposed. The series includes only a very resistant blue limestone and terminates abruptly along a well-marked and highly irregular erosion surface covered by almost a mile of volcanic material, chiefly lava flows. The character of the bottom of the section is likewise unknown, since it lies apparently far below the present level of erosion.

The Cretaceous limestones of the Cotahuasi Canyon are everywhere greatly and irregularly disturbed. Typical conditions are represented in the maps and sections, Figs. 166 and 167. They are penetrated and tilted by igneous masses, apparently the feeders of the great lava sheets that form the western summit of the Cordillera. From the restricted development of the limestones along a western border zone it might be inferred that they represent a very limited marine invasion. It is certainly clear that great deformative movements were in progress from at least late Palæozoic time since all the Palæozoic deposits are broken abruptly down in this direction, and, except for such isolated occurrences as the land Carboniferous at Pacasmayo, are not found anywhere in the coastal region today. The Cretaceous is not only limited within a relatively narrow shore zone, but also, like the Palæozoic, it is broken down toward the west, not reappearing from beneath the Tertiary cover of the desert region or upon the granite-gneisses that form the foundation for all the known sedimentary strata of the immediate coast.

From these considerations I think we have a strong suggestion of the geologic date assignable to the development of the great fault that is the most strongly marked structural and physiographic feature of the west coast of South America. Since the development of this fault is so intimately related to the origin of the Pacific Ocean basin its study is of special importance. The points of chief interest may be summarized as follows:

(1) The character of the land Carboniferous implies a much greater extent of the land than is now visible.

(2) The progressive coarsening of the Carboniferous deposits westward and their land derivation, together with the great thickness of the series, point to an elevated land mass in process of erosion west of the series as a whole, that is west of the present coast.

(3) The restricted development of the Cretaceous seas upon the western border of the Carboniferous, and the still more restricted development of the Tertiary deposits between the mountains and the present coast, point to increasing definition of the submarine scarp through the Mesozoic and the Tertiary.

(4) The Tertiary deposits are all clearly derived from the present mountains and have been washed seaward down slopes with geographic relations approximately like those of the present.

(5) From the great width, deep dissection, and subsequent burial of the Tertiary terraces of the coast, it is clear that the greater part of the adjustment of the crust to which the bordering ocean basin is due was accomplished at least by mid-Tertiary time.

Aside from the fossiliferous limestones of known Cretaceous age there have been referred to the Cretaceous certain red sandstones and shales marked, especially in the central portions of the Cordillera, by the presence of large amounts of salt and gypsum. These beds were at first considered Permian, but Steinmann has since found at Potosí related and similar formations with Cretaceous fossils. In this connection it is also necessary to add that the great red sandstone series forming the eastern border of the Andes in Bolivia is of uncertain age and has likewise been referred to the Cretaceous, though the matter of its age has not yet been definitely determined. In 1913 I found it appearing in northwestern Argentina in the Calchaquí Valley in a relation to the main Andean mass, similar to that displayed farther north. It contains fossils and its age was, therefore, readily determinable there.[53]

In the Peruvian field the red beds of questionable age were not examined in sufficient detail to make possible a definite age determination. They occur in a great and only moderately disturbed series in the Anta basin north of Cuzco, but are there not fossiliferous. The northeastern side of the hill back of Puqura (of the Anta basin: to be distinguished from Puquiura in the Vilcabamba Valley) is composed largely of rocks of this class. In a few places their calcareous members have been weathered out in such a manner as to show karst topography. Where they occur on the well-drained brow of a bluff the caves are used in place of houses by Indian farmers. The large and strikingly beautiful Lake Huaipo, ten miles north of Anta, and several smaller, neighboring lakes, appear to have originated in solution depressions formed in these beds.

The structural relation of the red sandstone series to the older rocks is well displayed about half-way between Urubamba and Ollantaytambo in the deep Urubamba Valley. The basal rocks are slaty schist and granite succeeded by agglomerates and basalt porphyries upon whose eroded surfaces (Fig. 169) are gray to yellow cross-bedded sandstones. Within a few hundred feet of the unconformity gypsum deposits begin to appear and increase in number to such an extent that the resulting soil is in places rendered worthless. Copper-stained bands are also common near the bottom of the series, but these are confined to the lower beds. Higher up in the section, for example, just above the gorge between Urubamba and Ollantaytambo, even-bedded sandstones occur whose most prominent characteristic is the regular succession of coarse and fine sandstone beds. Such alternations of character in sedimentary rocks are commonly marked by alternating shales and sandstones, but in this locality shales are practically absent. Toward the top of the section gypsum deposits again appear first as beds and later, as in the case of the hill-slope on the southern shore of Lake Huaipo, as veins and irregular masses of gypsum. The top of the deformed Cretaceous (?) is eroded and again covered unconformably by practically flat-lying Tertiary deposits.

TERTIARY

The Tertiary deposits of the region under discussion are limited to three regions: (1) the extreme eastern border of the main Cordillera, (2) intermontane basins, the largest and most important of which are (a) the Cuzco basin and (b) the Titicaca-Poopó basin on the Peruvian-Bolivian frontier, and (3) in the west-coast desert and in places upon the huge terraces that form a striking feature of the topography of the coast of Peru.

It has already been pointed out that the eastern border of the Cordillera is marked by a fault of great but undetermined throw, whose topographic importance may be estimated from the fact that even after prolonged erosion it stands nearly four thousand feet high. Cross-bedded and ripple-marked features and small lenses of conglomerate are common. The beds now dip at an angle approximately 20° to 50° northward at the base of the scarp, but have decreasing dip as they extend farther north and east. It is noteworthy that the deposits become distinctly conglomeratic as flatter dips are attained, and that there seems to have been a steady accumulation of detrital material from the mountains for a long period, since the deposits pass in unbroken succession from the highly indurated and massive beds of the mountain base to loose conglomerates that now weather down much like an ordinary gravel bank. In a few places just below the mouth of the Ticumpinea, logs about six inches in diameter were observed embeded in the deposits, but these belong distinctly to the upper horizons.

The border deposits, though they vary in dip from nearly flat to 50°, are everywhere somewhat inclined and now lie up to several hundred feet above the level of the Urubamba River. Their upper surface is moderately dissected, the degree of dissection being most pronounced where the dips are steepest and the height greatest. In fact, the attitude of the deposits and their progressive change in character point toward, if they do not actually prove, the steady and progressive character of the beds first deposited and their erosion and redeposition in beds now higher in the series.

Upon the eroded upper surfaces of the inclined border deposits, gravel beds have been laid which, from evidence discussed in a later paragraph, are without doubt referable to the Pleistocene. These in turn are now dissected. They do not extend to the highest summits of the deformed beds but are confined, so far as observations have gone, to elevations about one hundred feet above the river. From the evidence that the overlying horizontal beds are Pleistocene, the thick, inclined beds are referred to Tertiary age, though they are nowhere fossiliferous.

Observations along the Urubamba River were extended as far northward as the mouth of the Timpia, one of the larger tributaries. Upon returning from this point by land a wide view of the country was gained from the four-thousand-foot ridge of vertical Carboniferous limestone, in which it appeared that low and irregular strike ridges continue the features of the Tertiary displayed along the mountain front far northward as well as eastward, to a point where the higher ridges and low mountains of older rock again appear--the last outliers of the Andean system in Peru. Unfortunately time enough was not available for an extension of the trip to these localities whose geologic characters still remain entirely unknown. From the topographic aspects of the country, it is, however, reasonably certain that the whole intervening depression between these outlying ranges and the border of the main Cordillera, is filled with inclined and now dissected and partly covered Tertiary strata. The elevation of the upper surface does not, however, remain the same; it appears to decrease steadily and the youngest Tertiary strata disappear from view below the sediments of either the Pleistocene or the present river gravels. In the more central parts of the depression occupied by the Urubamba Valley, only knobs or ridges project here and there above the general level.

_The Coastal Tertiary_

The Tertiary deposits of the Peruvian desert region southwest of the Andes have many special features related to coastal deformation, changes of climate, and great Andean uplifts. They lie between the west coast of Peru at Camaná and the high, lava-covered country that forms the western border of the Andes and in places are over a mile thick. They are non-fossiliferous, cross-bedded, ripple-marked, and have abundant lenses of conglomerate of all sizes. The beds rest upon an irregular floor developed upon a varied mass of rocks. In some places the basement consists of old strata, strongly deformed and eroded. In other places it consists of a granite allied in character and probably in origin with the old granite-gneiss of the Coast Range toward the west. Elsewhere the rock is lava, evidently the earliest in the great series of volcanic flows that form this portion of the Andes.

The deposits on the western border of the Andes are excellently exposed in the Majes Valley, one of the most famous in Peru, though its fame rests rather upon the excellence and abundance of its vineyards and wines than its splendid geologic sections. Its head lies near the base of the snow-capped peaks of Coropuna; its mouth is at Camaná on the Pacific, a hundred miles north of Mollendo. It is both narrow and deep; one may ride across its floor anywhere in a half hour. In places it is a narrow canyon. Above Cantas it is sunk nearly a mile below the level of the desert upland through which it flows. Along its borders are exposed basal granites, old sedimentaries, and lavas; inter-bedded with it are other lavas that lie near the base of the great volcanic series; through it still project the old granites of the Coast Range; and upon it have been accumulated additional volcanic rocks, wind-blown deposits, and, finally, coarse wash formed during the glacial period. From both the variety of the formations, the small amount of marginal dissection, and the excellent exposures made possible by the deep erosion and desert climate, the Majes Valley is one of the most profitable places in Peru for physiographic and geologic study.

The most complete succession of strata (Tertiary) occurs just below Cantas on the trail to Jaguey (Fig. 171). Upon a floor of granite-gneiss, and alternating beds of quartzite and shale belonging to an older series, are deposited heavy beds of red sandstone with many conglomerate lenses. The sandstone strata are measurably deformed and their upper surfaces moderately dissected. Upon them have been deposited unconformably a thicker series of deposits, conglomerates, sandstones, and finer wind-blown material. The basal conglomerate is very coarse--much like beach material in both structure and composition, and similar to that along and south of the present coast at Camaná. Higher in the section the material is prevailingly sandy and is deposited in regular beds from a few inches to a few feet in thickness. Near the top of the section are a few hundred feet of strata chiefly wind deposited. Unconformably overlying the whole series and in sharp contrast to the fine wind-blown stuff below it, is a third series of coarse deposits about five hundred feet thick. The topmost material, that forming the surface of the desert upland, consists of wind-blown sand now shifted by the wind and gathered into sand dunes or irregular drifts, banks of white earth, “tierra blanca,” and a pebble pavement a few inches thick.

If the main facts of the above section are now summarized they will facilitate an understanding of other sections about to be described, inasmuch as the summary will in a measure anticipate our conclusions concerning the origin of the deposits and their subsequent history. The sediments in the Majes Valley between Cantas and Jaguey consist of three series separated by two unconformities. The lowermost series is evenly bedded and rather uniform in composition and topographic expression, standing forth in huge cliffs several hundred feet high on the eastern side of the valley. This lower series is overlain by a second series, which consists of coarse conglomerate grading into sand and ultimately into very fine fluffy wind-deposited sands and silts. The lower series is much more deformed than the upper, showing that the deforming movements of later geologic times have been much less intense than the earlier, as if there had been a fading out or weakening of the deforming agents. Finally there is a third series several hundred feet thick which forms the top of the section.

Three other sections may now be examined, one immediately below Cantas, one just above, and one opposite Aplao. The section below Cantas is shown in Fig. 173, and indicates a lower series of red sandstones crossed by vertical faults and unconformably overlain by nearly horizontal conglomerates, sandstones, etc., and the whole faulted again with an inclined fault having a throw of nearly 25°. A white to gray sandstone unconformably overlying the red sandstone is shown interpolated between the lowermost and uppermost series, the only example of its kind, however. No important differences in lithographical character may be noted between these and the beds of the preceding section.

Again just above Cantas on the east side of the valley is a clean section exposing about two thousand feet of strata in a half mile of distance. The foundation rocks are old quartzites and shales in regularly alternating beds. Upon their uneven upper surfaces are several thousand feet of red sandstones and conglomerates, which are both folded and faulted with the underlying quartzites. Above the red sandstones is a thick series of gray sandstones and silts which makes the top of the section and unconformably overlies the earlier series.

A similar succession of strata was observed at Aplao, still farther up the Majes Valley, Fig. 174. A greatly deformed and metamorphosed older series is unconformably overlaid by a great thickness of younger strata. The younger strata may be again divided into two series, a lower series consisting chiefly of red sandstones and an upper consisting of gray to yellow, and only locally red sands of finer texture and more uniform composition. The two are separated by an erosion surface and only the upper series is tilted regionally seaward with faint local deformation; the lower series is both folded and faulted with overthrusts aggregating several thousand feet of vertical and a half mile of horizontal displacement.

The above sections all lie on the eastern side of the Majes Valley. From the upper edge of the valley extensive views were gained of the strata on the opposite side, and two sections, though they were not examined at close range, are at least worth comparing with those already given. From the narrows below Cantas the structure appears as in Figs. 175-176, and shows a deforming movement succeeded by erosion in a lower series. The upper series of sedimentary rock has suffered but slight deformation. A still more highly deformed basal series occurs on the right of the section, presumably the older quartzites. At Huancarqui, opposite Aplao, an extensive view was gained of the western side of the valley, but the lower Tertiary seems not to be represented here, as the upper undeformed series rests unconformably upon a tilted series of quartzites and slates. Farther up the Cantas valley (an hour’s ride above Aplao) the Tertiary rests upon volcanic flows or older quartzites or the granite-gneiss exposed here and there along the valley floor.

In no part of the sedimentaries in the Majes Valley were fossils found, save in the now uplifted and dissected sands that overlie the upraised terraces along the coast immediately south of Camaná and also back of Mollendo. Like similar coastal deposits elsewhere along the Peruvian littoral, the terrace sands are of Pliocene or early Pleistocene age. The age of the deposits back of the Coast Range is clearly greater than that of the coastal deposits, (1) since they involve two unconformities, a mile or more of sediments, and now stand at least a thousand feet above the highest Pliocene (or Pleistocene) in the Camaná Valley, and (2) because the erosion history of the interior sediments may be correlated with the physiographic history of the coastal terraces and the correlation shows that uplift and dissection of the terraces and of the interior deposits went hand in hand, and that the deposits on the terraces may similarly be correlated with alluvial deposits in the valley.

We shall now see what further ground there is for the determination of the age of these sediments. Just below Chuquibamba, where they first appear, the sediments rest upon a floor of volcanic and older rock belonging to the great field now known from evidence in many localities to have been formed in the early Tertiary, and here known to be post-Cretaceous from the relations between Cretaceous limestones and volcanics in the Cotahuasi Valley (see p. 247). Although volcanic flows were noted interbedded with the desert deposits, these are few in number, insignificant in volume, and belong to the top of the volcanic series. The same may be said of the volcanic flows that locally overlie the desert deposits. We have then definite proof that the sandstones, conglomerates, and related formations of the Majes Valley and bordering uplands are older than the Pliocene or early Pleistocene and younger than the Cretaceous and the older Tertiary lavas. Hence it can scarcely be doubted that they represent a considerable part of the Tertiary period, especially in view of the long periods of accumulation which the thick sediments represent, and the additional long periods represented by the two well-marked unconformities between the three principal groups of strata.

If we now trace the physical history of the region we have first of all a deep depression between the granite range along the coast and the western flank of the Andes. Here and there, as in the Vitor, the Majes, and other valleys, there were gaps through the Coast Range. Nowhere did the relief of the coastal chain exceed 5,000 feet. The depression had been partly filled in early geologic (probably early Paleozoic) time by sediments later deformed and metamorphosed so that they are now quartzites and shales. The greater resistance of the granite of the Coast Range resulted in superior relief, while the older deformed sedimentaries were deeply eroded, with the result that by the beginning of the Tertiary the basin quality of the depression was again emphasized. All these facts are expressed graphically in Fig. 171. On the western flanks of the granite range no corresponding sedimentary deposits are found in this latitude. The sea thus appears to have stood farther west of the Coast Range in Paleozoic times than at present.

For the later history it is necessary to assemble the various Tertiary sections described on the preceding pages. First of all we recognize three quite distinct types of accumulations, for which we shall have to postulate three sets of conditions and possibly three separate agents. The first or lowermost consists of even-bedded deposits of red and gray sandstones, the former color predominating. The material is in general well-sorted save locally, where lenses and even thin beds of conglomerate have been developed. There is, however, about the whole series a uniformity and an orderliness in striking contrast to the coarse, cross-bedded, and irregular material above the unconformity. On their northeastern or inner margin the sandstones are notably coarser and thicker, a natural result of proximity to the mountains, the source of the material. The general absence of wind-blown deposits is marked; these occur entirely along the eastern and northern portions of the deposits and are recognized (1) by their peculiar cross-bedding, and (2) by the fact that the cross-bedding is directed northeastward in a direction contrary to the regional dip of the series, a condition attributable to the strong sea breezes that prevail every afternoon in this latitude.

The main body of the material is such as might be deposited on the wide flood plains of piedmont streams during a period of prolonged erosion on surrounding highlands that served as the feeding grounds of the streams. The alternations in the character of the deposits, alternations which, in a general view, give a banded appearance to the rock, are produced by successions of beds of fine and coarse material, though all of it is sandstone. Such successions are probably to be correlated with seasonal changes in the volume and load of the depositing streams.

To gain an idea of the conditions of deposition we may take the character of the sediments as described above, and from them draw deductions as to the agents concerned and the manner of their action.

We may also apply to the area the conclusions drawn from the study of similar deposits now in process of formation. We have between the coast ranges of northern Chile and the western flanks of the Cordillera Sillilica, probably the best example of piedmont accumulation in a dry climate that the west coast of South America affords.

Along the inner edge of the Desert of Tarapacá, roughly between the towns of Tarapacá and Quillagua, Chile, the piedmont gravels, sands, silts, and muds extend for over a hundred miles, flanking the western Andes and forming a transition belt between these mountains and the interior basins of the coast desert. The silts and muds constitute the outer fringe of the piedmont and are interrupted here and there where sands are blown upon them from the higher portions of the piedmont, or from the desert mountains and plains on the seaward side. Practically no rain falls upon the greater part of the desert and the only water it receives is that borne to it by the piedmont streams in the early summer, from the rains and melted snows of the high plateau and mountains to the eastward. These temporary streams spread upon the outer edge of the piedmont a wide sheet of mud and silt which then dries and becomes cracked, the curled and warped plates retaining their character until the next wet season or until covered with wind-blown sand. The wind-driven sand fills the cracks in the muds and is even drifted under the edges of the upcurled plates, filling the spaces completely. Over this combined fluvial and æolian deposit is spread the next layer of mud, which frequently is less extensive than the earlier deposits, thus giving abundant opportunity for the observation of the exact manner of burial of the older sand-covered stratum.

Now while the alternations are as marked in Peru as in Chile, it is noteworthy that the Tertiary material in Peru is not only coarse throughout, even to the farthest limits of the piedmont, but also that the alternating beds are thick. Moreover, there are only the most feeble evidences of wind action in the lowermost Tertiary series. I was prepared to find curled plates, wind-blown sands, and muds and silts, but they are almost wholly absent. It is, therefore, concluded that the dryness was far less extreme than it is today and that full streams of great competency flowed vigorously down from the mountains and carried their loads to the inner border of the Coast Range and in places to the sea.

The fact that the finer material is _sandy_, not clayey or silty, that it almost equals in thickness the coarser layers, and that its distribution appears to be co-extensive with the coarser, warrants the conclusion that it too was deposited by competent streams of a type far different from the withering streams associated with piedmont deposits in a thoroughly arid climate like that of today. Both in the second Tertiary series and on the present surface are such clear examples of deposits made in a drier climate as to leave little doubt that the earliest of the Tertiary strata of the Majes Valley were deposited in a time of far greater rainfall than the present. It is further concluded that there was increasing dryness, as shown by hundreds of feet of wind-blown sand near the top of the section. But the growing dryness was interrupted by at least one period of greater precipitation. Since that time there has been a return to the dry climate of a former epoch.

Uplift and erosion of the earliest of the Tertiary deposits of the Majes Valley is indicated in two ways: (1) by the deformed character of the beds, and (2) by the ensuing coarse deposits which were derived from the invigorated streams. Without strong deformations it would not be possible to assign the increased erosion so confidently to uplift; with the coarse deposits that succeed the unconformity we have evidence of accumulation under conditions of renewed uplift in the mountains and of full streams competent to remove the increasing load.

It is in the character of the sediments toward the top of the Tertiary that we have the clearest evidence of progressive desiccation of the climate of the region. The amount of wind-blown material steadily increases and the uppermost five hundred feet is composed predominantly, and in places exclusively, of this material. The evidences of wind action lie chiefly in the fine (in places fluffy) nature of the deposits, their uniform character, and in the tangency of the layers with respect to the surface on which they were deposited. There are three diagnostic structural features of great importance: the very steep dip of the fine laminae; the peculiar and harmonious blending of their contacts; the manner in which the highly inclined laminae cut off and succeed each other, whereby quite bewildering changes in the direction of dip of the inclined beds are brought about on any exposed plane. Some of these features require further discussion.

It is well known that the front of a sand dune generally consists of sand deposited on a slope inclined at the angle of repose, say between 30° and 35°, and rolled into place up the long back slope of the dune by the wind. It has not, however, been generally recognized that the angle of repose may be exceeded (a) when there exists a strong back eddy or (b) when the wind blows violently and for a short time in the opposite direction. In either case sand is carried up the short steep slope of the dune front and accumulated at an angle not infrequently running up to 43° and 48° and locally, and under the most favorable circumstances, in excess of 50°. The conditions under which these steep angles are attained are undoubtedly not universal, but they can be found in some parts of almost any desert in the world. They appear not to be present where the sand grains are of uniform size throughout, since that leads to rolling. They are found rather where there is a certain limited variation in size that promotes packing. Packing and the development of steep slopes are also facilitated in parts of the coastal desert of Peru by a cloud canopy that hangs over the desert in the early morning, that in the most favorable places moistens even the dune surfaces and that has least penetration on the steep semi-protected dune fronts. Sand later blown up the dune front or rolled down from the dune crest is encouraged to remain near the cornice on an abnormally steep slope by the attraction which the slightly moister sand has for the dry grains blown against it. Since dunes travel and since their front layers, formed on steep slopes, are cut off to the level of the surface in the rear of the dune, it follows that the steepest dips in exposed sections are almost always less than those in existing dunes. Exceptions to the rule will be noted in filled hollows not re-excavated until deeply covered by wind-blown material. These, re-exposed at the end of a long period of wind accumulation, may exhibit even the maximum dips of the dune cornices. Such will be conspicuously the case in sections in aggraded desert deposits. On the border of the Majes Valley, from 400 to 500 feet of wind-accumulated deposits may be observed, representing a long period of successive dune burials.

The peculiar blending of the contact lines of dune laminae, related to the tangency commonly noted in dune accumulations, is apparently due to the fact that the wind does not require a graded surface to work on, but blows uphill as well as down. It is present on both the back-slope and the front-slope deposits. Its finest expression appears to be in districts where the dune material was accumulated by a violent wind whose effects the less powerful winds could not destroy.

It is to the ability of the wind to transport material against, as well as with, gravity, that we owe the third distinct quality of dune material, the succession of flowing lines, in contrast to the succession of now flat-lying now steeply inclined beds characteristic of cross-bedded material deposited by water. One dune travels across the face of the country only to be succeeded by another.[54] Even if wind aggradation is in progress, the plain-like surface in the rear of a dune may be excavated to the level of steeply inclined beds upon whose truncated outcrop other inclined beds are laid, Fig. 178. The contrast to these conditions in the case of aggradation by water is so clearly and easily inferred that space will not be taken to point them out. It is also true as a corollary to the above that the greater part of a body of wind-drifted material will consist of cross-bedded layers, and not a series of evenly divided and alternating flat-lying and cross-bedded layers which result from deposition in active and variable currents of water.

The caution must of course be observed that wind action and water action may alternate in a desert region, as already described in Tarapacá in northern Chile, so that the whole of a deposit may exhibit an alternation of cross-bedded and flat-lying layers; but the former only are due to wind action, the latter to water action.

Finally it may be noted that the sudden, frequent, and diversified dips in the cross-bedding are peculiarly characteristic of wind action. Although one sees in a given cross-section dips apparently directed only toward the left or the right, excavation will supply a third dimension from which the true dips may be either observed or calculated. These show an almost infinite variety of directions of dip, even in restricted areas, a condition due to the following causes:

(1) the curved fronts of sand dunes, which produce dips concentric with respect to a point and ranging through 180° of arc; (2) the irregular character of sand dunes in many places, a condition due in turn to (a) the changeful character of the strong wind (often not the prevailing wind) to which the formation of the dunes is due, and (b) the influence of the local topography upon wind directions within short distances or upon winds of different directions in which a slight change in wind direction is followed by a large change in the local currents; (3) the fact that all combinations are possible between the erosion levels of the wind in successive generations of dunes blown across a given area, hence _any_ condition at a given level in a dune may be combined with _any other_ condition of a succeeding dune; (4) variations in the sizes of successive dunes will lead to further contrasts not only in the scale of the features but also in the direction and amount of the dips.

Finally, we may note that a section of dune deposits has a distinctive feature not exhibited by water deposits. If the foreset beds of a cross-bedded water deposit be exposed in a plane parallel to the strike of the beds, the beds will appear to be horizontal. They could not then be distinguished from the truly horizontal beds above and below them. But the conditions of wind deposition we have just noted, and chiefly the facts expressed by Fig. 178, make it impossible to select a position in which both tangency and irregular dips are not well developed in a wind deposit. I believe that we have in the foregoing facts and inferences a means for the definite separation of these two classes of deposits. Difficulties will arise only when there is a quick succession of wind and water action in time, or where the wind produces powerful and persistent effects without the actual formation of dunes.

The latest known deposits in the coastal region are found surmounting the terrace tops along the coast between Camaná and Quilca, where they form deposits several hundred feet thick in places. The age of these deposits is determined by fossil evidence, and is of extraordinary interest in the determination of the age of the great terraces upon which they lie. They consist of alternating beds of coarse and fine material, the coarser increasing in thickness and frequency toward the bottom of the section. It is also near the bottom of the section that fossils are now found; the higher members are locally saline and throughout there is a marked inclination of the beds toward the present shore. The deposits appear not to have been derived from the underlying granite-gneiss. They are distributed most abundantly near the mouths of the larger streams, as near the Vitor at Quilca, and the Majes at Camaná. Elsewhere the terrace summit is swept clean of waste, except where local clay deposits lie in the ravines, as back of Mollendo and where “tierras blancas” have been accumulated by the wind.

These coastal deposits were laid down upon a dissected terrace up to five miles in width. The degree of dissection is variable, and depends upon the relation of the through-flowing streams to the Coast Range. The Vitor and the Majes have cut down through the Coast Range, and locally removed the terrace; smaller streams rising on the flanks of the Coast Range either die out near the foot of the range or cross it in deep and narrow valleys. The present drainage on the seaward slopes of the Coast Range is entirely ineffective in reaching the sea, as was seen in 1911, the wettest season known on the coast in years and one of the wettest probably ever observed on this coast by man.

In consequence of their deposition on a terrace that ranges in elevation from zero to 1,500 feet above sea level, the deposits of the coast are very irregularly disposed. But in consequence of their great bulk they have a rather smooth upper surface, gradation having been carried to the point where the irregularities of the dissected terrace were smoothed out. Their general uniformity is broken where streams cross them, or where streams crossed them during the wetter Pleistocene. Their elevation, several hundred feet above sea level, is responsible for the deep dissection of their coastal margin, where great cliffs have been cut.

PLEISTOCENE

The broad regional uplift of the Peruvian Andes in late Tertiary and in Pleistocene times carried their summits above the level of perpetual snow. It is still an open question whether or not uplift was sufficiently great in the early Pleistocene to be influenced by the first glaciations of that period. As yet, there are evidences of only two glacial invasions, and both are considered late events on account of the freshness of their deposits and the related topographic forms. The coarse deposits--nearly 500 feet thick--that form the top of the desert section described above clearly indicate a wetter climate than prevailed during the deposition of the several hundred feet of wind-blown deposits beneath them. But if our interpretation be correct these deposits are of late Tertiary age, and their character and position are taken to indicate climatic changes in the Tertiary. They may have been the mild precursors of the greater climatic changes of glacial times. Certain it is that they are quite unlike the mass of the Tertiary deposits. On the other hand they are separated from the deposits of known glacial age by a time interval of great length--an epoch in which was cut a benched canyon nearly a mile deep and three miles wide. They must, therefore, have been formed when the Andes were thousands of feet lower and unable to nourish glaciers. It was only after the succeeding uplifts had raised the mountain crests well above the frost line that the records of oscillating climates were left in erratic deposits, troughed valleys, cliffed cirques and pinnacled divides.

The glacial forms are chiefly at the top of the country; the glacial deposits are chiefly in the deep valleys that were carved before the colder climate set in. The rock waste ground up by the ice was only a small part of that delivered to the streams in glacial times. Everywhere the wetter climate resulted in the partial stripping of the residual soil gathered upon the smooth mature slopes formed during the long Tertiary cycle of erosion. This moving sheet of waste as well as the rock fragments carried away from the glacier ends were strewn along the valley floors, forming a deep alluvial fill. Thereby the canyon floors were rendered habitable.

In the chapters on human geography we have already called attention to the importance of the U-shaped valleys carved by the glaciers. Their floors are broad and relatively smooth. Their walls restrain the live stock. They are sheltered though lofty. But all the human benefits conferred by ice action are insignificant beside those due to the general shedding of waste from the cold upper surfaces to the warm levels of the valley floors. The alluvium-filled valleys are the seats of dense populations. In the lowest of them tropical and sub-tropical products are raised, like sugar-cane and cotton, in a soil that once lay on the smooth upper slopes of mountain spurs or that was ground fine on the bed of an Alpine glacier.

The Pleistocene deposits fall into three well-defined groups: (1) glacial accumulations at the valley heads, (2) alluvial deposits in the valleys, and (3) lacustrine deposits formed on the floors of temporary lakes in inclosed basins. Among these the most variable in form and composition are the true glacier-laid deposits at the valley heads. The most extensive are the fluvial deposits accumulated as valley fill throughout the entire Andean realm. Though important enough in some respects the lacustrine deposits are of small extent and of rather local significance. Practically none of them fall within the field of the present expedition; hence we shall describe only the first two classes.

The most important glacial deposits were accumulated in the eastern part of the Andes as a result of greater precipitation, a lower snowline, and catchment basins of larger area. In the Cordillera Vilcapampa glaciers once existed up to twelve and fifteen miles in length, and those several miles long were numerous both here and throughout the higher portions of the entire Cordillera, save in the belt of most intense volcanic action, which coincides with the driest part of the Andes, where the glaciers were either very short or wanting altogether.

Since vigorous glacial action results in general in the cleaning out of the valley heads, no deposits of consequence occur in these locations. Down valley, however, glacial deposits occur in the form of terminal moraines of recession and ground moraines. The general nature of these deposits is now so well known that detailed description seems quite unnecessary except in the case of unusual features.

It is noteworthy that the moraines decrease in size up valley since each valley had been largely cleaned out by ice action before the retreat of the glacier began. Each lowermost terminal moraine is fronted by a great mass of unsorted coarse bowldery material forming a fill in places several hundred feet thick, as below Choquetira and in the Vilcapampa Valley between Vilcabamba and Puquiura. This bowldery fill is quite distinct from the long, gently inclined, and stratified valley train below it, or the marked ridge-like moraine above it. It is in places a good half mile in length. Its origin is believed to be due to an overriding action beyond the last terminal moraine at a time when the ice was well charged with débris, an overriding not marked by morainal accumulations, chiefly because the ice did not maintain an extreme position for a long period.

In the vicinity of the terminal moraines the alluvial valley fill is often so coarse and so unorganized as to look like till in the cut banks along the streams, though its alluvial origin is always shown by the topographic form. This characteristic is of special geologic interest since the form may be concealed through deposition or destroyed by erosion, and no condition but the structure remain to indicate the manner of origin of the deposit. In such an event it would not be possible to distinguish between alluvium and till. The gravity of the distinction appears when it is known that such apparently unsorted alluvium may extend for several miles forward of a terminal moraine, in the shape of a widespreading alluvial fan apparently formed under conditions of extremely rapid aggradation. I suppose it would not be doubted in general that a section of such stony, bowldery, unsorted material two miles long would have other than a glacial origin, yet such may be the case. Indeed, if, as in the Urubamba Valley, a future section should run parallel to the valley across the heads of a great series of fans of similar composition, topographic form, and origin, it would be possible to see many miles of such material.

The depth of the alluvial valley fill due to tributary fan accumulation depends upon both the amount of the material and the form of the valley. Below Urubamba in the Urubamba Valley a fine series is displayed, as shown in Fig. 180. The fans head in valleys extending up to snow-covered summits upon whose flanks living glaciers are at work today. Their heads are now crowned by terminal moraines and both moraines and alluvial fans are in process of dissection. The height and extent of the moraines and the alluvial fans are in rough proportion and in turn reflect the height, elevation, and extent of the valley heads which served as fields of nourishment for the Pleistocene glaciers. Where the fans were deposited in narrow valleys the effect was to increase the thickness of the deposits at the expense of their area, to dam the drainage lines or displace them, and to so load the streams that they have not yet cleared their beds after thousands of years of work under torrential conditions.

Below Urubamba the alluvial fans entering the main valley from the east have pushed the river against its western valley wall, so that the river flows on one side against rock and on the other against a hundred feet of stratified material. In places, as at the head of the narrows on the valley trail to Ollantaytambo, a flood plain has been formed in front of the scarp cut into the alluvium, while the edge of the dissected alluvial fans has been sculptured into erosion forms resembling bad-lands topography. On the western side of the valley the alluvial fans are very small, since they are due to purely local accumulations of waste from the edge of the plateau. Glaciation has here displaced the river. Its effects will long be felt in the disproportionate erosion of the western wall of the valley.

By far the most interesting of the deposits of glacial time are those laid down on the valley floors in the form of an alluvial fill. Though such deposits have greater thickness as a rule near the nourishing moraines or bordering alluvial fans at the lower ends of the valleys, they are everywhere important in amount, distinctive in topographic form, and of amazingly wide extent. They reach far into and possibly across the Amazon basin, they form a distinct though small piedmont fringe along the eastern base of the Andes, and they are universal throughout the Andean valleys. That a deposit of such volume--many times greater than all the material accumulated in the form of high-level alluvial fans or terminal moraines--should originate in a tropical land in a region that suffered but limited Alpine glaciation vastly increases its importance.

The fill is composed of both fine and coarse material laid down by water in steep valley floors to a depth of many feet. It breaks the steep slope of each valley, forming terraces with pronounced frontal scarps facing the river. On the raw bluffs at the scarps made by the encroaching stream good exposures are afforded. At Chinche in the Urubamba Valley above Santa Ana, the material is both sand and clay with an important amount of gravel laid down with steep valleyward inclination and under torrential conditions; so that within a given bed there may be an apparent absence of lamination. Almost identical conditions are exhibited frequently along the railway to Cuzco in the Vilcanota Valley. The material is mixed sand and gravel, here and there running to a bowldery or stony mass where accessions have been received from some source nearby. It is modified along its margin not only in topographic form but also in composition by small tributary alluvial fans, though these in general constitute but a small part of the total mass. At Cotahuasi, Fig. 29, there is a remarkable fill at least four hundred feet deep in many places where the river has exposed fine sections. The depth of the fill is, however, not determined by the height of the erosion bluffs cut into it, since the bed of the river is made of the same material. The rock floor of the valley is probably at least an additional hundred feet below the present level of the river.

Similar conditions are well displayed at Huadquiña, where a fine series of terraces at the lower end of the Torontoy Canyon break the descent of the environing slopes; also in the Urubamba Valley below Rosalina, and again at the edge of the mountains at the Pongo de Mainique. It is exhibited most impressively in the Majes Valley, where the bordering slopes appear to be buried knee-deep in waste, and where from any reasonable downward extension of rock walls of the valley there would appear to be at least a half mile of it. It is doubtful and indeed improbable that the entire fill of the Majes Valley is glacial, for during the Pliocene or early Pleistocene there was a submergence which gave opportunity for the partial filling of the valley with non-glacial alluvium, upon which the glacial deposits were laid as upon a flat and extensive floor that gives an exaggerated impression of their depth. However, the head of the Majes Valley contains at least six hundred feet and probably as much as eight hundred feet of alluvium now in process of dissection, whose coarse texture and position indicates an origin under glacial conditions. The fact argues for the great thickness of the alluvial material of the lower valley, even granting a floor of Pliocene or early Pleistocene sediments. The best sections are to be found just below Chuquibamba and again about halfway between that city and Aplao, whereas the best display of the still even-floored parts of the valley are between Aplao and Cantas, where the braided river still deposits coarse gravels upon its wide flood plain.