The Geologic Story of the Front Range
Colorado is treasured for its outdoor recreation and beautiful vistas. Another beautiful part of Colorado is its geology. While out hiking around Fort Collins, seeing a concert at Red Rocks Amphitheater, or driving I-70 into the mountains, people see geology all around them. Some may even question what those rocks are doing there, or why they’re tilted the way they are. The stories these rocks can tell is fascinating and complex, and the domain of geologists. Watch geologist Richard Newman introduce Front Range geology and discuss how the rocks beneath our feet can be storytellers.
The core basics of geology are routed in the main rock types: sedimentary, igneous, and metamorphic.
Sedimentary rocks are made of, you guessed it, sediments! Sand, dirt, fine particles like clay and ash, they settle into huge sheets, or layers, of rock in areas where the Earth is flat. These layers are simular to layers of a cake, where different types of sediment settle on top of one another, and eventually over time get compressed and lithified (just a fancy word for harden to a rock).
Igneous rocks are rocks that have formed from the cooling of magma (molten rock inside the earth) or lava (molten rock on the surface of the earth). These rocks can be very hard, like a granite countertop for example. Often, these hard rocks are the backbone of large mountain ranges, like the Rocky Mountains.
Metamorphic rocks are rocks that have been undergone a lot of heat and pressure, and have changed internally because of their intense past. However, if a rock gets too hot, or is under too much pressure, that rock will melt, and when it solidifies again it will be igneous.
Metamorphic rocks are complex, but because of their history of high temperature and pressure, they can be extremely hard and extremely old. The oldest rock on Earth is a metamorphic rock that is over 4 billion years old.
With a basic understanding of the rock types, we can now turn our attention to the story that is our dynamic planet. Earth is constantly changing its surface through the forces of weathering and erosion, and through the movement of the tectonic plates, which are HUGE slabs of the surface of the earth that slowly move around, bashing some continents together and tearing others apart. As these plates move vast distances, over hundreds of millions of years, the climate of an area changes. For example, roughly 300 million years ago, Colorado was tropical and was located near the equator!
Sedimentary rocks are record keepers of these climactic and tectonic changes. Their layers can be thought of as pages of a book that span the incomprehensible journey that is geologic time. In the Colorado Front Range, these layers are exposed for all to “read” and they tell a story that starts over 500 million years ago.
Listen to this podcast with James Hagadorn, a geologist for the Denver Museum of Nature and Science, talk about this intricate story.
So what is the “layer cake” of Front Range sediments? What do they look like and what do they represent in the story of Colorado? In geology, these layers are referred to as formations, and some formations can be summarized together as groups. See below for the chronological story of the major rock layers of the Front Range.
Fountain Formation
The Fountain Formation is the oldest sedimentary unit in the Front Range, and it tells the story of the erosion of the Ancestral Rocky Mountains. This formation is mostly a conglomerate, meaning it is made of large pebbles and sometimes cobbles, like what you would see in the Poudre River today.
The environment that this formation records is a high-energy fluvial (river) environment, and slopes of weathered debris that come from the once mighty Ancestral Rockies.
The Fountain Formation creates some of the Front Range’s most iconic outcrops, including Red Rocks, Garden of the Gods, and the Boulder Flat Irons.
All of these spectacular natural areas are the same unit. If you look closely next time you’re hiking around Garden of the Gods, or climbing up a Flat Iron, look for swaths of pebbles that are ancient stream channels filled with stones of a mountain range that has been completely erased from the surface of the Earth.
Ingleside and Lyons Formations
Continuing our story, the Ingleside and Lyons Formations are the next chapters in the geologic history of the Front Range, and represent a period of aridity in Colorado. The Ingleside is a transition of sorts between the fluvial environment of the Fountain and desert environment of the Lyons.
The Lyons Formation represents a large desert that was on the flank of the now aging Ancestral Rockies. This desert was characterized by large sand dunes. Today we can see impressive crossbedding (mini layers of sand) preserved in rock that is 250 million years old.
These formations are relatively strong and resistant to erosion which means they form a hogback, or a ridge that is present all along the Front Range. In Fort Collins, they create the ridge that is on the western side of Horsetooth Reservoir.
Morrison Formation
The Morrison Formation is a colorful variety of green, red, yellow, and white rocks. These are rocks are made of clay and other fine material called silt, and where laid down in a muddy tidal flat, similar to the Amazon flood plain today.
The Morrison is chock-full of dinosaurs. It is Jurassic in age, and the swampy conditions in which it was deposited were perfect for burying and preserving bones, many of which are now on display around the U.S.
Because these rocks are made of fine sediment, they are not very strong, or resistant to erosion. Because of this, in the Front Range this unit forms slopes coming off the topographic highs of the Dakota formation, which we’ll learn about next. In some areas, the Morrison also forms loose, rolling hills of sediment, like what is seen in Dinosaur National Monument.
Dakota Group
The Dakota group is dominantly made up of different sandstones, which not surprisingly, are rocks made of sand. The Dakota represents an ancient beach off the coast of a large interior seaway that was creeping into Colorado during the Cretaceous. By this time, the Ancestral Rockies had been reduced to rubble, and Colorado was relatively flat.
Sandstone is resistant to erosion, so the Dakota creates a major hogback throughout the Front Range. In the Fort Collins area, it is the ridge that is on eastern side of Horsetooth Reservoir.
Further south, it houses the famous dinosaur tracks of Dinosaur Ridge.
Niobrara Group
The Niobrara group has two members. The shale of the Smoky Hill member was deposited in calm waters of the interior seaway as it transgressed (or moved westwards). The Smoky Hill is know for its oil, and is an important part of the Colorado oil and gas industry.
The other member (called the Fort Hays) is made of limestone, which is formed in shallow, warm marine waters and is the compressed remains of tiny organisms that had calcium-carbonate shells, like coral. Because of this environment, there are many fossils of ocean-dwelling dinosaurs in the Niobrara.
The Smoky Hill is comprised of weak, fine sediments, and forms valleys. It is usually covered with soil and vegetation, and outcrops are rare. The Fort Hayes, however, is stronger and forms a small hogback just east of the large Dakota Formation ridge. If you are in Fort Collins and would like to find some fossils, you can go to Coyote Ridge Natural Area and walk out to the Fort Hayes limestone.
Pierre Shale
The Pierre Shale is a massive formation that underlays many Front Range cities, Denver, and much of the eastern plain of Colorado. This unit was formed by the continued transgression of the interior seaway that flooded eastern Colorado during the Cretaceous period. Shale is formed in deep, calm marine water, and is basically mud and silt turned to stone. Outcroppings of the shale are rare.
Tectonic Tilting
During the deposition of the Pierre Shale, tectonic collision off the coast of California generated stress deep into the interior of the continent, the middle of the U.S. responded by crumpling. Old, hard, igneous and metamorphic rocks that were buried under the sediments were lifted up, creating the Rocky Mountains we know and love today. This same uplift tilted and folded the sedimentary rocks of the Front Range, leaving them in their current position.
