Part 5

[13] The exact age of the iron and manganese deposits here referred to is, in some cases, a little uncertain. Some may be Cambrian, others Silurian, but the exact determination of the age of the horizon is not a part of the present discussion. The matter has been discussed by the writer in Geological Survey of Arkansas, 1890, Vol. I., pp. 376–379.

[14] See Geological Survey of Arkansas, 1890, Vol. I., pp. 320–325.

[15] See Geological Survey of Arkansas, 1890, Vol. I., pp. 456–457.

[16] R. D. Irving and C. R. Van Hise, U. S. Geol. Survey, Tenth Ann. Report, 1888–1889, Vol. I, pp. 409–422.

[17] Geol. Survey of Penn., Vol. II, 1858, p. 739.

[18] The solutions may be precipitated, as already shown, either with or without admixture with mechanical sediments; and there are in nature all gradations from almost pure deposits of iron and manganese ore to beds of shale, sandstone, etc. stained with iron or manganese. Subsequent concentration frequently causes decided changes in the latter deposits (see p. 370).

[19] It has been suggested by A. A. Julien (Proceed. Amer. Assoc. Adv. Sci., Vol. XXVIII., 1879, p. 356) that in some cases the carbonates of iron and manganese may be only the fixed residue of organic compounds of more complex form once in solution in surface waters.

[20] This oxide is generally in the form of the peroxide or the sesquioxide in a more or less hydrous condition.

[21] Jahrb. des Vereins f. Naturkunde in Herz. Nassau, Vol. VI., p. 160 (Bischof).

[22] Marcasite has the same composition as pyrite, but differs in crystalline form.

[23] Manganese also occurs in the mineral youngite, which contains lead, zinc, iron, manganese and sulphur, but the mineral is considered of doubtful homogeneity. (See System of Mineralogy, E. S. Dana, 1892).

[24] When manganese is precipitated artificially as sulphide it is usually in the form of the monosulphide (MnS), in either a hydrous or an anhydrous form.

[25] Manganese occurs in various hydro-silicates, but they do not appear to be deposited as sedimentary strata in the same manner as glauconite.

[26] If the water moved very slowly, the deposition would probably take place approximately in the same spot; if the waters moved more rapidly, the iron might be deposited in one place and the carbonate in another, in the way explained on page 363.

[27] The hydrous oxides of iron are not crystalline.

[28] See p. 363.

[29] Bischof suggests that the action described by Fresenius causes the separate deposition of iron and manganese; and also that it explains the occurrence of large deposits of manganese ore in regions where the iron ore contains least of that ingredient. (See Elements of Chemistry and Phys. Geol., Vol. III., pp. 531–532.)

[30] In some cases these iron and manganese deposits are undoubtedly formed by the replacement of limestone or other rocks, as is further discussed on pages -- to --.

[31] Kentucky Geol. Survey, Report of Progress, Vol. III., New Series, 1877, p. 164.

[32] U. S. Geol. Survey, Tenth Annual Report, 1888–1889, Vol. I., pp. 409–422.

SOME RIVERS OF CONNECTICUT.[33]

_Outline._--Introduction.--Topography of Connecticut: The upland plateau, its origin, date, elevation, valleys sunk beneath its surface.--Lowland on the Triassic area.--Later oscillations.-- Résumé of the topography.--Early drainage.--Re-adjusted streams. --Revived streams.--Unconformable rivers, consequent or superimposed.--Pleistocene changes; the Farmington, Quinnipiac, Scantic.--Abandoned gaps.

_Introduction._ In order to study intelligently the history of a river, one must first become acquainted with the present physical geography of the region in which the river lies, and know the stages of its development. Therefore, before classifying the rivers of Connecticut, I shall consider the topography of the state, and in a few paragraphs outline the successive cycles in the history of its growth. The scope of this article will not permit a discussion or even a full statement of the evidence on which these conclusions are based. They have been stated at considerable length by Professor W. M. Davis,[34] and the reader is referred to his papers for the complete discussion. His conclusions in respect to the physical geography are accepted here without question, and form the basis for the discussion on the rivers of the state.

_Topography of Connecticut._ Connecticut can be said to consist of two great areas quite distinct in topography and geologic structure.[35] On the east and on the west are the crystalline uplands which rise from sea level along the Sound to 1,700 and 1,800 feet in the northwestern part of the state, and to 600 and 700 feet in the northeastern. These uplands consist chiefly of gneiss and granite, probably of pre-Paleozoic age, which are now much folded, faulted and crumpled. Between these two areas of crystallines is a lowland belt of Triassic sandstone and shale, twenty to twenty-five miles wide, extending from New Haven north through the center of the state and including in its borders New Haven, Meriden, Hartford, New Britain and many towns of lesser note. These sandstones form a monocline with an eastward dip of 10° to 30°, and in addition to being tilted they have been faulted since their deposition in a shallow, slowly-subsiding trough of crystallines. Their thickness is variously estimated--3,000 to 5,000 feet, Dana; 10,000 or more, Davis. This lowland is interrupted by a series of trap ridges, which in general present steep faces toward the west, whereas their eastward slope is gradual, less than the dip of the sandstones.

_The upland plateau._ Suppose we ascend the highest point of these trap ridges, the old tower on Talcott Mt., nine miles west of Hartford; we are 900 feet above the sea level and more than 600 above the plain at our feet. A few miles to the west across the sandstone valley, rise the crystalline uplands, which extend far to the north and to the south. On the east across the Connecticut we see the eastern uplands. The first impression, which comes to one as he gazes upon these uplands and which is strengthened with each view, is that few hills rise above the general level of the plateau; the crest line is nearly horizontal, declining gently to Long Island Sound. Above this general level are a few rounded domes, but no sharp, towering peaks. Below it valleys have been cut, but they do not destroy the plateau-like appearance. A view from the western plateau across the sandstone valley shows the remarkably even crest line of the trap ridges, a crest line which approximates in height the uplands on the east and west. A nearer view of the upland corroborates our first impressions of the gently rolling character of the inter-stream surfaces, but we have a better view of the valleys which have been sunk beneath the general level and of the low rounded hills which rise above it. In popular parlance the country is “hilly.” It is uneven, not because there are high hills, but rather because there are deep valleys. If in imagination we fill up these valleys and the wide Triassic lowland to the general level of the broad inter-stream surfaces, we shall have constructed a gently undulating plateau, dipping to the south and east--a peneplain.[36]

_Origin of the peneplain._ This is not a constructional surface, for the rocks are greatly tilted, folded and faulted, so that the surface consequent upon such disturbance must have been complex and mountainous. Long subaërial denudation upon a folded and faulted mass when the land stood much lower than at present produced this plateau. Evidently it could be produced by denudation only at or near baselevel, for the effect of erosion upon a mass high above baselevel is to accentuate its topographic relief, not to reduce it. We naturally ask ourselves, “At what stage in geologic history did this denudation occur?”

_Date of the peneplain._ The erosion which accomplished this great work must have commenced after the formation and dislocation of the Triassic beds, for the even crest line of the trap ridges, a part of which--perhaps all--were contemporaneous with the sandstones, is a part of the dissected peneplain; but to fix the date of the completion of the peneplain, we must turn to evidence presented in New Jersey.[37] There we learn that by the close of Cretaceous times, the country was eroded nearly to baselevel, and we may therefore speak of the relative position of the land and sea, to which the land was at this time reduced, as the Cretaceous baselevel, and this land surface as the Cretaceous peneplain.

_Elevation of the peneplain._ In post-Cretaceous, presumably early Tertiary[38] times, the land was elevated to nearly its present height and remained at that altitude, so far as topographic evidence shows, during Tertiary times. The proofs of this elevation are the valleys which the streams have sunk below the general level. That this was not a simple uplift, but was accompanied with tilting and warping, is clear from the following considerations. The depth to which a stream can cut its valley depends directly upon its height above baselevel. If the present surface were a peneplain uniformly elevated, the head waters and middle courses of a river would not be cut so deep in the surrounding plain as its lower course. But the reverse is true of the rivers of Connecticut. The depth of the valley increases inland, being greater in those regions where the peneplain was raised the highest. A comparison of the upper and lower valleys of the Housatonic, Naugatuck, Quinnebaug, and of the Connecticut at Middletown, where it enters the plateau, and at its mouth, will give some idea of the amount of the warping. It will not give an _exact measure_ of it for several reasons: first, the upper courses of the rivers have not yet reached the present baselevel; second, the present altitude of the uplands is the result of the post-Cretaceous uplift and warping, plus a probable later post-Tertiary uplift (to be mentioned later), besides several minor oscillations, the last of which was downward, and is recorded near the coast in the drowned condition of the rivers. As has been already said, the peneplain is highest in the northwest, and gradually declines to sea level toward the south and east.

_Consequences of the uplift._ The consequences of this uplift are seen in the valleys, which are cut into the peneplain, and which have destroyed the level character of the country. In the hard crystalline rocks the valleys are generally narrow and deep, with bold slopes;[39] where they are cut in the crystalline limestone, they are wider and more open. In marked contrast, however, is the lowland on the Triassic area in which only the trap ridges remain to tell of the former altitude of the general surface, and the immense amount of erosion which has taken place on the soft sandstones and shales. Indeed erosion has progressed so rapidly on these soft rocks, that they have been worn down almost to a new baselevel in the same length of time in which the hard crystallines have been only trenched. This fact cannot be too strongly emphasized. The broad sandstone lowland from New Haven north into Massachusetts has been carved out of the uplifted peneplain in soft rocks, during the same time in which the Connecticut has excavated its gorge in the crystallines below Middletown, and the Housatonic has opened its upland valley on the limestones. The difference in results is due not to a difference of time, but to the difference in the relative hardness of the rocks.

On the basis of this principle the age of certain river gorges to which reference will be made later can be fixed. The _narrow_ passage of the Quinnipiac through a sandstone ridge southwest of Meriden cannot belong to the same cycle of erosion as the _broad sandstone lowland_ on either side of it, but manifestly must be much younger. So, also, the narrow passage of the Farmington at Tariffville, where it crosses the trap ridge through a gorge free from drift, is of much later date than the _broader_ valley more or less encumbered with drift which the upper part of the same river has cut in the hard crystalline schists. Cook’s Gap in the trap sheet west of New Britain is much broader than either of the above, and belongs to the Tertiary cycle of erosion, although as I shall endeavor to show later, it was probably not occupied by a stream during the whole cycle. In marked contrast, also, with the Tariffville gorge is the gap by which the Westfield river in Massachusetts cuts the trap ridge. This gap was formerly broad and open--the result of Tertiary erosion--but is now filled with drift, in which the river is at present working. Since these two rivers are essentially the same in size, are now at the same level, and the rock is the same in both cases, the only explanation for the difference in the two passages is that they belong to different cycles.

To recapitulate, the results of the post-Cretaceous uplift are seen in the valleys which have been cut in the peneplain. The narrow valleys in the gneisses and schists, the upland valleys in the limestones, the wide open, drift encumbered gaps in the trap ridge,--Cook’s and the Westfield river gaps,--the broad open lowland on the sandstones, are all the result of erosion in this cycle. The Quinnipiac gorge in the sandstone, and the Tariffville gorge in the trap are just as surely of a later date. They do not at all accord with the work of the earlier cycle either in size, angle of slope, or depth.

This conclusion is somewhat at variance with an opinion expressed by Professor J. D. Dana,[40] but it seems justifiable in view of the successive cycles in the physical development of the region. In another part of this article I shall consider these gaps again in connection with their river histories, and shall give additional reasons why I venture to differ from so eminent an authority.

_Length of this cycle._ This cycle of erosion beginning with the post-Cretaceous uplift was not so long as the preceding cycle. In the earlier one the whole state was reduced to a peneplain; in the later cycle only the soft Triassic sandstones were brought near to baselevel. It probably lasted through Tertiary times, and was brought to a close by a slight uplift. The result of this uplift is well shown in Pennsylvania[41] and New Jersey.[42] It is not well shown in Connecticut, but there seem to be some traces of it in the trenches the rivers have cut below the level of the sandstone peneplain. However, these trenches are so much obscured by drift that a positive statement is not warranted. It may, however, be spoken of provisionally as the post-Tertiary uplift. There may have been later oscillations of small amount, probably were; here and there are shreds of evidence which point to such oscillations, but only one movement has had an effect upon the topography, which can be recognized. The fjorded condition of all the rivers along the Sound--the Norwalk, Saugatuck, New Haven bay, Niantic and Thames are the best examples--shows that within comparatively recent time there has been a slight subsidence of the land. But this movement is not to be compared in amount with those of the earlier cycles.

_The drift._ Over all the state in varying thickness lies the glacial drift, either in its typical unmodified development as till, or in its modified form, as river terraces, kames, eskers and sand-plains. It is of importance in this connection only as it has affected the topography of the country and so modified the drainage. Examples of these modifications will be mentioned later.

_Résumé._ There was first a long cycle of denudation in pre-Triassic times, during which the contorted crystallines were worn down to a comparative level; second, a cycle of subsidence, deposition and volcanic outburst, during which the sea entered the crystalline trough, and the Triassic conglomerates, sandstones and shales were deposited with the intercalated layers of lava; third, a long cycle of elevation, folding, faulting and erosion, during which the sedimentary beds were elevated--tilted into the present faulted monocline, and this constructional surface worn down to a baselevel of erosion in late Cretaceous times. Each of these cycles probably represents the sum total of several subordinate cycles. There was, fourth, a post-Cretaceous uplift inaugurating a period of erosion lasting through Tertiary times and resulting in the formation of valleys in the hardest rocks, and a lowland approaching baselevel on the Triassic sandstones and shales; fifth, a probable late or post-Tertiary uplift, when the valleys were deepened and the lowlands trenched--obscure in Connecticut, but well shown farther south; sixth, the land, near the coast at least, is now slightly lower than it has been in the not remote past, as is shown by the fjords.

With the changes of the physical geography clearly in mind, the rivers of Connecticut may now be examined in respect to their conditions of origin, the number of cycles through which they have lived, and the approach they have made to mature old age. But at the very outset a serious difficulty is encountered, for the geological structure of the state is nowhere well described, nor have topographic maps of all the districts yet been issued. Since the structural details are to some extent unknown it is unwise in many cases to attempt more than tentative conclusions. Several of the problems to be presented cannot be considered as settled. Considerable progress toward a final settlement will have been made, however, if the conditions of the problems are made clear, various hypotheses suggested, and the attention of workers in this field called to these questions.

_Early drainage._ Of the drainage of Connecticut during Jurassic and Cretaceous times very little can be said. It is not even known whether it was consequent upon the Jurassic tilting and faulting, or whether these deformations were so slow in their movement that the rivers persisted in spite of them. It may have been that the larger rivers were victorious, while the smaller were conquered and compelled to assume new consequent courses. Whatever was their origin there must have been abundant opportunities during the long erosion which resulted in the Cretaceous baselevel, and again in the period of revived and quickened degradation succeeding the post-Cretaceous uplift, for the streams to adjust themselves in a large degree to the geological structure. The contrast of hard and soft beds and the great elevation must have been potent factors in bringing to pass such a result. We expect to find the streams so far re-adjusted as to render improbable the discovery of their manner of origin.

_The Housatonic, a re-adjusted stream._ The best example of re-adjustment is found in the northwestern part of the state where the Housatonic and some of its branches follow well adjusted courses. From its headwaters, near Pittsfield, Mass., to New Milford, Conn., it has nearly all the way chosen its course along the Cambrian crystalline limestones in preference to the harder granites and gneisses on either side. The stratigraphical relationships of the limestone are not fully understood, but they seem to be deeply eroded anticlines and synclines, whose axes plunge north or south at various angles. The course of the river, if the drainage was consequent, was at first along the synclinal valleys, passing from one to another across the lowest points in the anticlinal ridge between them. But by a series of changes[43], resulting from the differential rates of erosion as hard or soft beds became exposed, the river previously to the Cretaceous baseleveling, seems to have re-adjusted its course to the softer limestones. However, there are several places where this conformity to structure does not seem to be the law; where the river departs from a limestone valley to flow for a time in the crystallines, only to return to the limestone again. The most marked instance of this is in the towns of Sharon and Cornwall, where the river leaves the limestone valley, which continues to the southwest, and flows for ten miles in a narrow gorge in the gneiss, only to again enter at its northern end a long narrow bed of limestone. The following seems to be the probable explanation. When the land stood at the elevation represented by the Cretaceous peneplain, these hard beds were below or but very slightly above baselevel, and were therefore undiscovered by the stream or had just begun to make themselves known late in the cycle. Had they been reached early in the cycle, when the stream was far above baselevel and presumably before many of its tributaries had been developed, and when it was therefore a smaller river, it is quite probable that further re-adjustments would have occurred, and the stream been led away from the hard rocks onto the softer beds to the west; but when they were reached the stream had cut so deeply and so nearly to baselevel that it was safe from capture. After the elevation of the peneplain the stream was revived and disclosed more and more of these hard beds, but was then, owing to the development and head-water growth of its tributaries, too important a river to be diverted by any rival. A river of this kind may be said to be “conformably superimposed” in distinction to one which is superimposed from an unconformable cover.

_Revived streams._ It is important to recognize the effect of the post-Cretaceous uplift upon the rivers at that time established. As the land was baseleveled and the velocity of the streams decreased, they lost in large degree their cutting power and sluggishly meandered more or less in broad flood-plains. During and for a period after the uplift, their cutting power was restored to them by virtue of their increased velocity and they excavated the deep narrow valleys which we find in the crystalline highlands. The upper course of the Housatonic is a good example of a river re-adjusted to the structure during one cycle, revived by uplift to a second cycle of erosion, and in places “conformably superimposed” upon structures from which it would have been led away in the ordinary course of re-adjustment. Its tributaries, the East Aspetuck, Still, Shepaug, and Pomeraug follow courses re-adjusted in one cycle and revived in a later uplift.

We can assert with the more confidence that such was the history of the upper Housatonic, because we find in other states, in regions whose history has been the same, similar examples of “conformably superimposed” and “revived” streams. The Musconetcong and Pequest, highland rivers of New Jersey, are streams “revived” from mature old age to vigorous youth and “conformably superimposed” upon saddles of gneiss between two limestone valleys.[44]