Part 5

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.

Second in importance among the animals of the period were the brachiopods or lamp-shells, not true mollusks although they were provided with similar shells composed of calcium phosphate or calcium carbonate. Shells are of two parts (bivalved) as in the case of clams, but the valves are above and beneath the body instead of on the right and left sides, which is the arrangement among mollusks. Although abundant as individuals, there were only a few species during the earlier part of the period; the number of species increased, however, and the race became very persistent. About seven thousand species have been described, and the race is not yet extinct although the number of living species is relatively small.