It is characteristic of earth history that the younger the rocks are, the more we know about them. This is because younger rocks lie near the surface, have not been disturbed as much by mountain building processes as have older rocks, and have not been affected as strongly by repeated erosion. Many of the events of the Cenozoic Era are documented in detail in the geology of Colorado, and these events have intimately influenced the scenery as we see it today.
The Cenozoic is the Age of Mammals. How it happened that mammals triumphed over reptiles is one of the mysteries of geology. Some scientists think that climatic changes—dropping temperatures and increases in rainfall—swung the balance in favor of the warm-blooded mammals. Others believe that cosmic ray bombardment during some unusual astronomical event may have destroyed many surface-living dinosaurs, while small burrowing mammals (as well as many small reptiles) were able to survive. Still others maintain that the superior intelligence and regulated body temperatures of mammals enabled them to win out in the battle for survival without the aid of climatic or cosmic change.
The names Tertiary and Quaternary, used for the two Cenozoic Periods, are holdovers from early studies in geology in which rocks were divided into Primary (very hard, crystalline rocks such as igneous and metamorphic rocks), Secondary (well consolidated layered rocks), Tertiary (layered rocks which are not fully cemented but which are nevertheless fairly well consolidated), and Quaternary (sediments in which the grains have not become cemented together).
Tertiary Period
(3-70 million years ago)
During the first part of the Tertiary Period, uplift began in earnest in Colorado and adjacent states. This uplift was part of the great Laramide Orogeny that built the Rocky Mountain chain from Alaska to New Mexico. The entire area rose above the level of the sea, and mountains were thrust up in a great series of north-south ranges that extended unbroken almost the length of the continent. Between the ranges, thick layers of gravel and sand, derived from the surrounding highlands, were deposited in intermontane basins. Occasional freshwater limestones and shales indicate the presence of lakes.
In Colorado, many details of the formation of the Rockies stand out in bold relief. The Front Range moved upward sharply, mostly as a linear block broken or faulted along both edges. Paleozoic and Mesozoic sediments along the margins of the block were steeply tipped and in some places even overturned, while in some localities Precambrian rocks were thrust out over the younger sediments.
Just east of the Front Range, especially in the area around Denver, the land remained lower and was the site of thick deposits of gravel and sand eroded from the range. The Denver Formation, the Arapahoe [Conglomerate], and the Dawson Arkose are more than 2,000 feet thick in this area. These are delta and river sediments, all varying a great deal from place to place. Individual layers of sand or gravel are not continuous over extensive areas, but some, such as the Castle Rock Conglomerate, are very prominent locally.
On Wolford Mountain, just north of Kremmling, Precambrian [granite] lies on top of Cretaceous shale. The older rocks were thrust up and over younger rocks during the Laramide Orogeny. The position of the [fault] shows clearly because trees prefer the granite soil above the fault to the shale below. (Jack Rathbone photo)
Along the eastern margin of the Front Range west of Castle Rock and Sedalia, rocks deposited at this time are now folded steeply, indicating that the mountains continued to rise even as basin sediments were being deposited.