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.
[Illustration: 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.]
[Illustration: 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.
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.
* * * * * * * *
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.