Only part of the earth’s very early history is represented in Colorado,
where the oldest known rocks are the gneisses and schists of the Idaho
Springs Formation, at least 1,800,000,000 years old. These rocks appear
to be the remains of ancient sediments, folded and metamorphosed into
vast mountain areas long before recognizable life inhabited the earth.
Precambrian rocks in Colorado are on the whole very poorly known. They
have, however, been studied in detail in the Front Range west of Denver
and Boulder, where they have been intensively explored for valuable
minerals. The lack of fossils in the oldest rocks makes their close
correlation difficult, but from studies of radioactive minerals
contained in these rocks, and of the relationships of the rock units
themselves, we can list them in order of their relative ages.
Note that the rock sequence given below reads chronologically from
bottom to top—a logical pattern in geology since younger rocks,
especially those of sedimentary origin, normally lie above older ones.
Recent studies indicate that the sequence may be much more complex than
shown here.
(youngest) Silver Plume Granite: light pinkish gray, fine-grained
granite.
Pikes Peak Granite: pink, coarse-grained granite.
Boulder Creek Granite: dark gray, faintly banded
granodiorite.
Coal Creek Quartzite: light gray quartzite with grains
ranging in size from fine sand to
boulders, with some interbedded
schist.
Swandyke Hornblende Gneiss: dark gray to black, strongly banded
gneiss.
(oldest) Idaho Springs Formation: gray to black schist and gneiss.
From a sequence such as this, it is possible to reconstruct some
features of Colorado’s early history. The first chapter of which we have
a record is the deposition of the Idaho Springs Formation, probably as
an accumulation of mud, sand, and limy mud in an ancient sea. Swandyke
deposition followed—the sediments were iron-rich, perhaps derived from
ancient volcanic materials. The original Coal Creek sediments were sands
and gravels, some of them quite coarse and therefore indicative of
near-shore deposition. The schist layers suggest that muds must have
been deposited also.
[Illustration: South of Ouray, Cambrian sandstones of the Sawatch
Formation lie almost horizontally across the vertical Precambrian
metamorphic rocks. (Jack Rathbone photo)]
Together these three formations represent some 40,000 feet of
sedimentary layers. Deposition of such a great thickness of mud, sand,
and lime must have taken a very long period of time. Details of the
geography of the continent during that period have of course been
obscured by later events, when these rocks were subjected to repeated
uplift, crumpling, folding, various degrees of remelting and
recrystallization, and erosion. But the ancient sediments must have been
derived from even more ancient highlands, either folded and faulted
mountains or volcanoes, and probably were deposited under water in broad
expanses of sea that covered portions of the continent.
[Illustration: Geologic map of Colorado. Geologic maps show the age of
rocks appearing at the surface, disregarding soil cover. A more detailed
geologic map of Colorado may be obtained from the U.S. Geological Survey
at the Federal Center in Denver.]
PRECAMBRIAN
PALEOZOIC
MESOZOIC
CENOZOIC SEDIMENTS
CENOZOIC VOLCANICS
Yampa River
White River
Fort Collins
South Platte River
Glenwood Springs
Denver
Colorado River
Grand Junction
Aspen
Gunnison River
Colorado Springs
Gunnison
Salida
Dolores River
Rio Grande
Arkansas River
La Junta
Walsenburg
Alamosa
Durango
The Boulder Creek, Pikes Peak, and Silver Plume Granites cut through the
metamorphic rocks, and are therefore younger. They represent pulses of
molten rock forced upward from deep within the crust, probably during
three separate episodes of mountain building. As each set of mountains
was formed, it was worn down, perhaps to low rolling hills, perhaps to
flat plains almost at sea level, and partially or entirely covered with
thick layers of sediment. Each time, another mountain building episode
followed, with new intrusions of granite and new metamorphism of the
pre-existing rocks.
Each succeeding period of metamorphism and mountain building further
changed the nature of the rocks involved, complicating the patterns of
folding and faulting, adding recrystallization to recrystallization,
until the oldest of the rocks bore little trace of their original
sedimentary nature. In general, the rocks that are oldest were most
altered by the repeated metamorphism, while the younger rocks were less
altered.
[Illustration: The Black Canyon of the Gunnison River is one of the
state’s deep and spectacular chasms. Canyon walls are of Precambrian
gneiss intruded by many dikes and highly fractured by later uplifts. The
flat upper surface of the Precambrian rocks represents an ancient plain
on which, during Jurassic time, the dinosaur-bearing Morrison Formation
was deposited. (John Chronic photo)]
The Precambrian Era ended with a long period of erosion, a period known
to geologists as the Lipalian Interval. During this time, over almost
the entire world there was no mountain building. The land lay sleeping,
subject only to the forces of erosion. The last mountains were flattened
nearly to sea level. Slow, sluggish streams and rivers carried sand and
mud toward the oceans—oceans in which perhaps primitive, soft-bodied
organisms, with no hard parts to be preserved as fossils, were beginning
to evolve.
On the continents, the time of intense metamorphism was over; most rocks
of later eras are preserved today in pretty much their original state.
The boundary between the Precambrian and later rocks is normally well
defined. It is visible at many places in Colorado: in Williams Canyon
near Colorado Springs, in Glenwood Canyon, near Red Rocks west of
Denver, just west of La Veta Pass, at the top of Royal Gorge and the
Black Canyon of the Gunnison. At most of these localities it is a
smoothly beveled surface, with highly contorted Precambrian rocks below
it and flat-lying Paleozoic sediments above it. Near Red Rocks and La
Veta Pass, the same relationship prevails, but the entire contact, and
the rocks above and below it, have been steeply tilted by the uplift of
the present mountains.
In portions of western North America, deposition late in Precambrian
time has left a series of flat-lying rocks between the contorted
Precambrian and later Paleozoic sediments. These rocks can be seen in
northwestern Colorado, where they form the dark red sedimentary core of
the Uinta Mountains.