Part 3

The whole of geological time has been divided and subdivided according to varying practices. The development of life is perhaps the one outstanding feature of the time divisions, but for the most part the changes in floras and faunas have been gradual rather than abrupt, and this makes it very difficult to draw sharp lines or to visualize beginnings and endings of the various stages of development. Occasionally there is good excuse for drawing a line, where the record is broken and resumed again after a long lapse of time. The principal cause of such breaks is the elevation of great land masses, which brings on an interval of erosion and surface destruction for the areas uplifted.

These movements of parts of the earth’s crust have been exceptionally pronounced at certain times, often culminating in the production of mountain systems, and because of the extreme changes they introduce are known as revolutions. The major divisions of prehistoric time have been established, at least in part, by such _revolutions_; crustal, climatic, or other _disturbances_, on a smaller scale and recurring with greater frequency, may be regarded as establishing boundaries for the minor divisions. Hence we have five great Eras of geological history, and these are divided again into Periods. The time chart shows an arrangement commonly used in America. In the first column the names of the Eras are stated in technical form. Closely coinciding with these terms are the popular names of the Ages which appear in the second column. These names, describing the dominant life of each age, are very convenient. The more scientific terms used for the eras, while serving essentially the same purpose, are a little more systematic and generalized in that they refer to ancient life (Paleozoic), middle life (Mesozoic), and recent life (Cenozoic), without being specific as to any class of animals or plants for any one division of time.

The period names, in the central column, have been derived from miscellaneous sources, some of them from geographical districts, some from descriptive references to prominent features of the rocks, others indicating a degree of approach to recent time. In paleontology (fossil study) it has long been a practice to cut the periods into lower, middle, and upper divisions, and in a few cases it has been found desirable to make two periods out of an old one. What was once known as the Lower Carboniferous is now commonly recognized as the Mississippian period while the upper portion has become the Pennsylvanian. The Lower Cretaceous is now the Comanchean of some authors.

Both old and new practices are responsible for a little confusion at the present time. A former division into Primary, Secondary, Tertiary, and Quaternary eras has been partly abandoned, but the term “Quaternary” still applies to the Age of Man, while “Tertiary time” remains in good usage for the balance of the Cenozoic era. Among the newer introductions may be mentioned the use of a Paleocene period which precedes the Eocene. Geologists are not entirely in agreement as to the necessity for this addition and many would not give it equivalent rank with other periods. In the interest of simplicity these modern refinements have been omitted from the chart.

The figures appearing in the third column, between the Ages and Periods, indicate the millions of years that have elapsed up to present time. They denote the age of the rocks at the beginning of each period. The age of a plant or animal which lived in Eocene time would be, according to this scale, somewhere between 35 million and 60 million years. In practice it is usually possible to determine whether a fossil was embedded in the rocks during an early or late portion of the period, and thus its age may be established within a shorter range, but it is impossible to be exact, even in terms of millions of years, with regard to anything as far back in prehistory as the Eocene period.

The period in which we are living today is known as Recent. It began at the close of the Ice Age or Pleistocene period about ten thousand years ago and represents so little of earth history since the beginning of life that a chart many times the length of this page would be required to show the rest of the periods in proportion. The Cambrian period is an early chapter in which the story of prehistoric life suddenly becomes clear and richly varied. It is, however, much farther from the beginning of the record than it is from the present, and the Pre-Cambrian eras would require a great deal more space in order to show their relative lengths. The Archeozoic and Proterozoic eras have to some extent been divided into periods, but the great antiquity of the rocks has obscured much of their history, and divisions established for one locality have been of little service elsewhere. Consequently, the period names are in less general use and the common practice is to refer to all this great stretch of time as Pre-Cambrian.

In the last column, at the right of the chart, some of the historical features are indicated. This column should be read from bottom to top in order to get the proper development of the story, and at best this sketchy outline of events can be no more than suggestive of the progress and decline through which the earth’s inhabitants have passed.

Rocks of every period except probably the Silurian are known to have been deposited somewhere in the Colorado area, although in most cases the record for each period is far from complete. Formations are too numerous and too varied locally to be shown on a chart of this type.

THE GEOLOGICAL SECTION

In the study of fossils there are two important field aids usually available. For any locality there should be a geological map and a section showing the sequence and character of the strata. On a small-scale map many of the local details have to be omitted, but the position of the larger exposures is indicated and, with this information at hand, the fossil-bearing strata may be located with the help of a geological section. The latter is frequently obtained from technical reports published by State and National Geological Surveys. Frequently, however, it is possible to obtain only a general plan for a given locality, and a great deal of literature may have to be scanned in order to get that. Excellent geological maps of Colorado have been published by the Colorado Geological Survey and the United States Geological Survey.

It often happens that a formation is not where we expect to find it, this being due to several possible factors. The sediments may not have been deposited there, or they may have been removed by erosion. Where the structure has been disturbed by folding and faulting, a multitude of complications is introduced. The expected sequence is sometimes inverted and repeated through a series of folds. Formations also may be moved miles out of place by faulting. Both thickness and character of sediments may vary considerably within a formation. In some regions the geology is very simple, in others extremely difficult to understand.

PERIODS

RECENT PLEISTOCENE PLIOCENE MIOCENE OLIGOCENE EOCENE CRETACEOUS THICKNESS SOFT SANDSTONES GRITS & CLAYS DENVER & ARAPAHOE 2000 ft. SANDSTONES, SHALES & LIGNITE LARAMIE 1000 ft. YELLOWISH SANDS & SHALES FOX HILLS 1000 ft. SOFT DARK GRAY OR RUSTY SHALE PIERRE 5000 ft. LIMESTONES & SHALES NIOBRARA 500 ft. DARK SHALES & LIME BENTON 400 ft. GRAY OR BUFF SANDSTONES & CLAYS DAKOTA 300 ft. SHALES, SANDSTONE & LIME MORRISON 200 ft. JURASSIC TRIASSIC PERMIAN DEEP-RED SANDY SHALES, LIME, GYPSUM LYKINS 700 ft. CARBONIFEROUS MASSIVE PINK OR WHITE SANDSTONE LYONS 200 ft. RED OR BROWN SANDSTONE & FOUNTAIN 1500 ft. CONGLOMERATE DEVONIAN SILURIAN ORDOVICIAN CAMBRIAN PRE-CAMBRIAN METAMORPHIC & INTRUDED ROCKS IDAHO SPRINGS SCHIST, GNEISS, QUARTZITE (PART) BASEMENT ROCKS of IGNEOUS ORIGIN

A generalized section for the western part of the Denver Basin is introduced here for the use of local students. The formations normally present in this region are shown in their usual position. They are briefly described on the chart, and their thickness is indicated by figures which may be regarded as near the maximum for the district. The section will apply to most of the foothills area between Morrison and Boulder though surface features and thickness of beds will vary considerably from place to place.

Certain of the formations are known to be fossil bearing, others barren or nearly so. When fossils are present they are usually restricted to certain localities, and these may be widely scattered. The following remarks apply to the possibilities for finding fossils in the formations named.

_Denver and Arapahoe._

Leaf impressions of palms, ferns, and numerous species of well-known trees and shrubs are common in many localities. Petrified wood is fairly abundant, and a few scattered bones of reptiles and mammals have been found. The two formations are treated as a unit because the Arapahoe is neither conspicuous nor sharply defined. Denver beds are well exposed on the slopes of Table Mountain at Golden; fossils, however, have been obtained from several localities nearer the city of Denver, notably from the hills just west of Overland Park.

_Laramie._

Plant material is locally abundant, principally the leaves of familiar deciduous trees, palms, and ferns. Many of the clay pits being worked near Golden are in this formation. Oysters and a few other mollusks may be found in some places.

_Fox Hills._

Better exposures of this formation are located to the north of Denver. Marine mollusks are most frequently found.

_Pierre._

In addition to the characteristic dark shales, this formation includes some limy material and sandstone beds, both of which are fossiliferous in places. Two types of marine mollusks are characteristic: _Inoceramus_, generic name for several species of clam-like bivalves readily identified by concentric elevations which produce a rippled effect on the shell surfaces; and _Baculites_, cephalopods with straight, chambered shells which often break at the suture lines, where the fossil is weakened by the chamber walls. Small oyster shells are fairly common also. The formation is to be found some distance to the east of the prominent hogback where it weathers into smooth surfaces in the form of broad valleys and flats, with rounded contours on the few elevations that may be present. It forms a soft, flaky soil when dry, is a sticky “gumbo” when wet. The clay is generally of a rather dark grayish color when freshly exposed but it takes on a rusty appearance after weathering. At various levels there are numerous iron-cemented concretions, many of which contain fossil shells.

_Niobrara._

The formation contains fossils rather similar to those of the Pierre. Shark’s teeth have been found in some of the lower beds. Limestone is a prominent feature, often forming a well defined ridge near the foot of the eastern slope of the main hogback. The limestones commonly have a chalky character.

_Benton._

The formation is not especially productive in this region. Marine shells are numerous in some localities, and bones of marine reptiles have been found at various places. As usually seen, it is almost entirely composed of impure clay shales, very dark, brownish-gray to almost black, and commonly interbedded with thin patches of white bentonite, yellow ochre, gypsum, and limestone.

_Dakota._

This formation produces the high hogback which is usually present some distance east of the Red Rocks. There are generally two or three layers of massive, light-colored sandstone separated by clays which are used extensively in the making of bricks and pottery. Leaf impressions and some fish scales are found in the clays and occasionally in the sandstone. The hogback is a good marker from which to locate other formations, because of its prominence in the foothills landscape.

_Morrison._

Good dinosaur material has been taken from the Canon City and Morrison districts. The formation is to be found on the lower west slope of the Dakota hogback. It consists of continental deposits of the stream and lake types. There is considerable sandstone in this formation and a little limestone is to be found here and there, but the most characteristic feature is in the shales. When freshly exposed, the shales are delicately tinted with gray, green, and maroon, a bronze-green being rather prominent. This formation is highly variable in character, with much of the clay often buried under the valley floor. In addition to the bones of reptiles, there are plant fossils, usually of poor quality, and fresh-water gastropods more or less abundant in some localities.

_Lykins._

Outcrops are not prominent, owing to the small amount of weather-resisting materials. The sandy clays are commonly of a deep red color mottled with spots of light gray. A white limestone is sometimes present near the middle of these deposits, and gypsum beds are included locally. The formation is often indicated only by red soil in the depressions between ridges. Few fossils have been reported.

_Lyons._

This formation is usually prominent as the eastern wall of the uplifted Red Rocks series. In some localities it forms a ridge of pink or white sandstone distinctly separated from the older sediments to the west. Very few fossils are found.

_Fountain._

Exposures usually are brown to red in color, though sometimes a dirty white. The prominent rocks are rather coarse sandstone, commonly with a gritty texture due to the angular character of the sand or gravel from which they were made. These are the westernmost of the Red Beds and the oldest of the uplifted sedimentary rocks bordering the foothills in most of our area. Fossils have been found in the formation, but it is practically barren for the territory here considered.

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This geological section also illustrates a method of dating crustal movements and the birth of mountain ranges, for the folding of the strata along the flanks of the Rocky Mountains has a great deal of significance in this connection. The sedimentary layers were originally deposited over much of the present mountain area in a horizontal position, and only those formations in existence at the time could be distorted by the upheavals which produced the new elevations. Of the series generally involved in the movement the Laramie beds are the youngest. Since these beds had not been formed until near the close of the Cretaceous period it is to be assumed that the mountains must be of more recent date, younger than the topmost of the deformed beds and at least as old as the lowermost of the undisturbed formations overlying them.

Some disturbance is evident also in the Arapahoe and Denver beds which overlie the Laramie, but this is believed to have occurred sometime after the occasion of the first great uplift. Volcanic materials in these beds lead to the belief that the sediments were deposited during a period of volcanic activity brought on by the crustal folding which terminated the Mesozoic era. Hence the conclusion arises that the age of the Denver and Arapahoe beds must coincide closely with some of the earlier stages in the history of the mountain system. This interval is often referred to as Post-Laramie time.

BEFORE THE AGE OF REPTILES

THE PRE-CAMBRIAN COMPLEX

The rocks of Pre-Cambrian time have been buried deeply under the accumulation of younger sediments, and the resulting pressure in many places has been tremendous. In addition to the effects of pressure there also is recorded in these ancient formations the repeated movements of the materials since they were first deposited. Vertical and side adjustments of parts, with relation to other parts, have distorted the original arrangement of the rock particles to such an extent that ordinary fossils would eventually become unrecognizable. These crushing, grinding, and kneading forces working through millions of years alone would account for the absence of fossils from the older deposits. Frequently the rocks have become so changed in form that their original character can only be conjectured, and because of this change they are known as metamorphic rocks.

A few beds of Archeozoic age remain in nearly their original condition, but they are either without fossils or they have produced very questionable and unsatisfactory specimens. The existence of life during these early stages of earth history is indicated largely by chemical rather than fossil evidence. Much of the ancient limestone has been converted into marble, but it is not unreasonable to believe that plants and animals were instrumental in the production of this type of rock as they are today. Certain varieties of iron ore deposits are now being built up by the aid of plants, and similar ores in the ancient rocks may have had a like origin. The presence of great quantities of carbon, in the form of graphite, may be regarded also as a sign of life, for this substance is accumulated on a large scale by living plants, and may be retained in a solid form after the partial decay of the plant tissues.

So far as the direct evidence goes, there is no sign of any creature of large size or of such complicated structure as the common plants and animals of today. The chemistry of the mineral deposits is not entirely convincing as to the presence of life, but it is regarded as highly probable that microscopic, single-celled plants and animals, comparable to modern algae and protozoa, were in existence during Archean time. Throughout later eras there is unmistakable evidence of gradual development from simpler to more elaborate life-forms and the Archeozoic is commonly regarded as a time of preparation during which simple organisms of some kind were becoming adapted to early conditions which could not support life on a higher plane. The importance of the work done by such lowly creatures in the preparation of suitable environments for more advanced modes of living is overlooked almost entirely.

During the next era, the Proterozoic, the record of life becomes somewhat clearer. Fossils are hardly to be regarded as abundant but there were several well-defined types of animals which left shells and other parts composed of mineral matter. Among these may be mentioned the Radiolaria, Foraminifera, Bryozoa, and Sponges. Radiolaria produced delicate, often lace-like shells of many patterns adorned with the radiating filaments or spines which have suggested the name for this group. Foraminifera produced minute shells, sometimes many chambered, and often bearing a confusing resemblance to the work of snails. Common chalk is composed almost entirely of such shells and fragments of them.

Sponges and Bryozoa are animals of slightly higher organization. They are many-celled instead of one-celled and the cells have special work to perform, which is a most important step in the direction of the specialization which characterizes the structural and life pattern of later arrivals. The Bryozoa lived in moss-like colonies which have been important rock-makers; the fossil forms bear some resemblance to corals. Sponges are too well known to require description although the familiar article of commerce is merely the framework of once-living animals. They represent the earliest organization of true animal bodies even though in appearance they may have a resemblance to plants.

Actual plants of this era were of the algae class, aquatic in habit as were their animal neighbors, the first to leave a record in the form of fossils. This record, obscure and distorted, has long been a source of perplexity to investigators. Without well-defined floras and faunas to guide them, and with rocks frequently in chaotic relationships, early geologists were content to regard it all as a “Pre-Cambrian complex.” Recent studies have contributed a great deal of information not available some years ago. It is quite possible that more advanced types of life were in abundance before the close of the second era, but material on which to base sound opinion is still scarce.

Rocks of Pre-Cambrian age are plentiful in the foothills region west of Denver. The schists, gneisses, and quartzites exposed for some miles immediately beyond the red-beds are part of this great complex. The Idaho Springs formation is known to be one of the oldest in this district, although its exact age has not been determined. Other formations are recognized among the metamorphic rocks of the region but none has contributed to our knowledge of early life.

CAMBRIAN LIFE

There can be no mistake as to the prolific development of life in Cambrian seas, for fossils of this age are to be found in many parts of the world, where ancient sea bottoms now form part of the land surface. Invertebrate animals appear to have made much progress, but plants were either scarce or too small and delicate to be productive of fossils. It is probable, however, that seaweeds and other algae were flourishing along with the invertebrates, because animal life is directly or indirectly dependent on the existence of plants. The latter sustain themselves by taking carbon and nitrogen from air, water, and soil, but animals must obtain their requirements by eating plants or eating each other. They cannot obtain what they need from the inorganic world without this help from the vegetable kingdom.

One group of animals stands out prominently above all its contemporaries. Known as the trilobites they were by far the most distinguished and most characteristic of Cambrian invertebrates. Trilobites inhabited the warmer seas of this period and several later ones, but were extinct by the end of the Paleozoic era. Hundreds of species have been described, most of them under four inches in length. Well-known distant relatives now living are the shrimps, and other crustaceans. The name Trilobite has reference to the three lobes which are apparent in the form of the upper surface, the central lobe forming a broad ridge extending along the back. Beneath the outer lobes on each side there was, during life, a row of short, jointed legs used for swimming and walking, but these delicate appendages are seldom preserved in the fossils.