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.
FORMATIONS of the DENVER-FOOTHILLS REGION
A GENERALIZED SECTION SHOWING SOME OF THE SURFACE FEATURES
RED BEDS HOGBACKS TABLE MOUNTAIN DENVER DISTRICT
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 & CONGLOMERATE | FOUNTAIN | 1500 ft. | ||
| DEVONIAN | ||||
| SILURIAN | ||||
| ORDOVICIAN | ||||
| CAMBRIAN | ||||
| PRE-CAMBRIAN | ||||
| METAMORPHIC & INTRUDED ROCKS SCHIST, GNEISS, QUARTZITE | IDAHO SPRINGS (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 âgumboâ 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.
Marine Beds of the Benton Formation, Northeastern Colorado
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.
Plesiosaur Bones in Place
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. Sharkâs 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.
* * * * * * * *
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.