Part 4

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.

REGION OF MOUNTAIN-MAKING UPLIFT

Formations bordering the mountains have been bent into upright positions.]

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.

A stream channel has cut deeply into the formation, uncovering and partly destroying a plesiosaur skeleton which was found at the level where the men are standing.]

Benton formation. Surface rubble has been cleared away, and several vertebrae are partially uncovered in the area at the right of the hammer.]

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.