The Cretaceous Sea - Late Cretaceous - just prior to the Laramide Orogeny (85 mya)
To tell a tale of any historical event takes sound records and a keen interpretative approach. And invariably, the older the event, the harder to tell the tale because the evidence is older, less reliable, not as prolific. Such is the challenge to the geologist; to tell the tale of the Earth and how it has changed and evolved as a planet and as part of the solar system, galaxy, and Universe.
But these geological tales put much of the burden of trust on the reader, as they must understand how science works and that science is a particular way of observing the world as it is and as it may have been. Geology uses the Principle of uniformitarianism, a big word which means that what we see in matter behavior today is what we would have seen in the past.
The age of the Earth is based on sound science that is accepted by the overwhelming majority of Earth scientists. This body of scientific evidence puts the creation of our universe about 13-15 billion years ago (bya) and our planet at nearly 4.6 bya and the matter source for our planet as the remnants of a super nova prior to 4.6 bya. Where the universe came from is still open to interpretation.
Our current view of Earth is stratified, with a solid and extremely dense inner core, dense liquid outer core, a less dense plastic-like mantle, and a thin crust made of lighter continental material and heavier oceanic material which floats on a slowly flowing mantle. It is generally accepted that the new Earth differentiated into layers in its early existence based on the density of the matter.
Theories about how the first rocks formed range from formation out of a magmatic surface, to bombardment of floating solid meteorites. Whatever the process, the first rocks to form are probably close to 4 billion years old. But because the Earth is dynamic and the surface is reshuffled and recycled, the oldest material has undergone the greatest loss and is less abundant.
The cores of most continents have quantities of rock dating from 4 to 3 billion years old and are referred to as cratons. In North America, the Canadian Shield is the craton rock. This shield is centered in the Hudson Bay area of Canada and extends into some areas of the upper Mid West and New England. Surrounding the craton is younger material, perhaps 2.5 bya, referred to as the North American Platform. The Platform extends the Shield farther south under the Great Plains sediments in the northern middle of North America. There is a very small outcrop of this older rock in Colorado, in the far northwestern corner of the state.
Most of what we think of as Colorado was added to the North American platform in a series of sedimentary depositions and subsequent metamorphism, and the intrusion of granitic rock in several distinctive phases. As we get within a half billion years of the present, a massive explosion of life bathes the Earth, and the younger sedimentary rocks are more abundant.
Colorado's oldest rocks are at the core of the Rocky Mountains, and include metasediments and igneous formations from about 1.8 to 1.4 bya. Here west of Fort Collins, the metasediments are dated at 1.71 bya, and various intrusions of igneous rocks at 1.7 and 1.4 bya. There then exists a great void in the rock record for this area.
Colorado has had numerous plate tectonic activities, from subduction areas to rifting events, which helped contribute to North American Platform growth and even reduction. About 300 million years ago, tectonic activity helped create what is known as the "Ancestral Rocky Mountains." Frontrangia, as it is referred to, became the source for sediments which gathered at its base, creating the Fountain Formation we see resting on the older metamorphic and igneous rocks to the west. These deposits continued as Pangaea split, and a great sea formed across the middle of North America. The seas retreated as the present Rocky Mountain thrust upward during the Laramide Orogeny lasting from 70 to 28 million years ago. More erosion deposited debris and gravels across the plains, and a last uplift 5 million years ago helped expose the Tertiary pediment and erode out the gravels to expose the late Cretaceous rocks of 65 million years ago. In the area of the Horsetooth Quadrangle, the youngest sedimentary rock, besides the river gravel, is the Cretaceous Pierre Shale.
The Cretaceous Sea - Late Cretaceous - just prior to the Laramide Orogeny (85 mya)
For a more complete and easy to read description, the first chapter of Roadside Geology of Colorado, 2nd Edition, is an excellent place to start. The Virtual Field Trip describes several of the shallow sea sedimentary rock deposits you will encounter as well as some sedimentary deposits formed as dunes in deserts.