Geologic History

Geology of Florissant, CO

The geologic history of Florissant Fossil Beds National Monument and the surrounding area is one that includes magmatic intrusions, volcanic eruptions, formations of a lake, and erosion. These geologic events and processes occurred over the course of one billion years. Geologists are able to interpret the geologic history of the Florissant region by studying the rocks that resulted from these events.

Geologic map of the Florissant region with Florissant Fossil Beds National Monument boundary in green.
Pikes Peak Granite
The oldest rocks found in the Florissant area are granites that intruded into the Earth’s crust between 1.1-1.8 billion years ago. Pikes Peak Granite (1.1 billion years old), the only granite found in Florissant Fossil Beds National Monument, makes up a structure called a batholith, which forms when a large mass of magma is emplaced deep underground and hardens. The Pikes Peak Batholith stretches about 25 miles wide and 80 miles long.

Outcrop of Pikes Peak Granite with rock hammer for scale.
Wall Mountain Tuff
The Florissant region was uplifted when the modern Rocky Mountains formed (70-40 million years ago), and then eroded away to a landscape of rolling hills over the span of millions of years. By the late Eocene, streams had cut valleys into the surface and volcanoes were also a dominant force in Colorado.  Around 36.7 million years ago, a large caldera collapse far west of Florissant produced an extremely hot and fast flow of ash and debris that blanketed the surrounding area, including the Florissant region, for tens of miles. This flow filled valleys and cooled and compacted to form a welded tuff known as the Wall Mountain Tuff. In the Florissant valley, this welded tuff was then eroded by streams. Today, only scattered outcrops of the Wall Mountain Tuff remain along the margins of the valley.

Outcrop of Wall Mountain Tuff with rock hammer for scale.
Florissant Formation
The Florissant Formation is made up of a variety of sedimentary rock types (e.g., mudstone, conglomerate, and shale) and associated volcanic eruptive products that were deposited 34 million years ago in an ancient lake environment, with intermittent stream episodes, within the Florissant valley. Almost all of the fossils discovered in the Florissant fossil beds have been found in the thin, lake-deposited paper-shales of the Florissant Formation.  More than 1800 species of fossil plants, insects, spiders, fish, birds, and mammals have been discovered within the Florissant Formation. 

The development of ancient Lake Florissant began with the influence of volcanism. About 18 miles southwest of Florissant Fossil Beds National Monument are the remains of the Guffey volcanic center. Similar in structure to Mt. St. Helen’s of the Cascade Range today, the Guffey stratovolcano sent flows of lava and debris down into the Florissant valley during the late Eocene. On at least two occasions, lahars (volcanic mudflows) from the Guffey volcanic complex intersected the stream of the ancient Florissant valley and created a natural dam, allowing for a lake to form. It was in this environment that paper-thin layers of ash, clay, and diatoms (algae) built up on the bottom of the lake and eventually compacted into fossil-rich shale containing the abundant fossil plants and insects. 

The lower shale unit of the Florissant Formation marks the first recorded lake episode of the ancient Florissant valley. As the lake grew and began to fill with sediment, the dam was eventually breached. The area then once again became a stream valley. Stream deposits of this time contain fossils of mammals, such as brontotheres (an animal that looked like a rhinoceros), Mesohippus (a small horse), and small mammals (e.g., rodents and rabbits) which appear in the lower mudstone unit. 

Another large eruption from the Guffey volcano triggered more lahars, one of which dammed the stream once again. On another occasion, a lahar flowed into the valley and buried the stumps of the redwood trees that grew there, forming a rock layer at the top of the lower mudstone unit. Visitors today can see the petrified redwood stumps.
The second episode of ancient Lake Florissant is evident by the middle shale unit. Once more, layers of ash, clay, and diatom mats accumulated as ash from new eruptions of the Guffey volcanic center entered the lake. This unit also contains many fossil leaves and insects. This second period of paper-shale deposition was interrupted when a large debris flow flowed into Lake Florissant, stirring up the sediment. This event is preserved as the caprock conglomerate. However, the debris flow was not large enough to fill the lake entirely and periodic volcanic eruptions led to new layers of shale (upper shale unit).

The rock record indicates that Lake Florissant disappeared after a huge eruption that covered the landscape with pumice. A cloud of erupted material fell into the lake and is preserved as the pumice conglomerate, the youngest and uppermost rock unit in the Florissant Formation. Any evidence of further eruptions has been erased by erosion.

Outcrops of the shale units of the Florissant Formation weather rapidly, as evident in this photo.

Stratigraphic column of the Florissant Formation. Rocks below the lower shale unit include Wall Mountain Tuff (hot pink) and Pikes Peak Granite (light pink).
Pleistocene gravels
The youngest rock unit observed in Florissant Fossil Beds National Monument is Pleistocene Ice Age gravels. The gravels are composed of eroded Pikes Peak Granite from the slopes of the surrounding hills that were then deposited in the valley floor. This material, called grus or colluvium, is a few feet deep and covers much of the Florissant Formation. Mammoth bones were discovered near the surface in this rock unit and radiocarbon dating of a tooth yielded an age of more than 43,000 years old. Although not presented in this database, you can learn more about the mammoth bones uncovered at Florissant Fossil Beds National Monument by visiting

Pleistocene gravels at the surface with rock hammer for scale.