Florissant Fossil Beds National Monument - Geology

Today Florissant Fossil Beds National Monument is a wonderland of meadows, forests, and wildflowers. Yet 34 million years ago, a description would tell another tale. Lake Florissant, streching 12 miles through an ancient forested valley, dominates the scene. Lush ferns and shrubs thrive under towering redwoods, cedars, pines, and a colorful mixed hardwood forest of maples, hickories, and oaks. In this warm temperate climate thousands upon thousands of insects crawl, fly, and buzz about. Fish, mollusks, birds and mammals inhabit the lake and it"s shores. Nearby, a volcano rumbles. In the past, volcanic mudflows blanketed parts of the forest burying trees and blocking a drainage which surrounds Lake Florissant. The volcano continues to erupt. The exploding volcano showers the countryside with millions of tons of ash, dust, and pumice. Ash falls directly into the lake or is washed in from the surrounding landscape. The ash weathers to clay at the bottom of the lake. Fragments of life (fish, insects, leaves, twigs, flowers, pollen, etc.) were trapped in a layer of volcanic sediments at the bottom of the lake. Eventually, these finely layered sediments become shale and transformed the buried plant and animal life into fossils. Today, Florissant Fossil Beds National Monument preserves this site. The fossil beds are internationally renowned for the variety and number of fossils - particularly of insects and plants - they have yielded since their discovery in the late 1800s. Paleontologist have collected more than 60,000 specimens for museums and universities around the world. These fossils reveal, in remarkable detail, what life of so long ago was like. Ever so fragile and tiny a creature as a butterfly may be preserved as a fossil that clearly shows its antennae, legs, hairs, and the pattern of its wings. Massive petrified redwood stumps are evidence that ancient life here had its giants, too. Yet, little remains of other ancient Lake Florissant life. Fossil bones, teeth, shells, and feather impressions reveal the existence of mollusks, fish, birds, opossums, mesohippus (an ancestor of the modern horse), and oreodonts (extinct pig-like animals). But unless a mammal or bird actually died in or near the lake, its chance of preservation were very slim. Future scientific explorations promise to unearth more of Florissant"s "buried treasures."

Rocks of the Florissant Area

Five different rock units occur in the Park at Florissant. The Florissant formation includes six layered members. The following descriptions are presented from youngest to oldest.

1. Quaternary (10,000 yrs. to present)]

Alluvium - fine to coarse grained sands, silts, and bog soils; found along stream drainages.

Slope wash - coarse grained detrital fragments of Pikes Peak Granite; forms steep slopes and low hills at base of eroded granite and at edges of valleys in the Florissant Basin.

2. Eocene (55 to 34 m.y.)

Florissant formation (33-34 m.y.)

a. Upper tuff

b. Upper tuff and interfingering lake shales

c. Lake shales

d. Lower tuff and interfingering lake shales

e. Lower tuff

f. Basal arkosic breccia

The Lake Shales (c.) are described as units from top to bottom:

a. coarse, conglomeritic sandstone

b. fine-grained, soft, yellowish-white sandstone, with bands that are more or less argillaceous, and containing fragments and stems of leaves

c. coarse gray and yellowish sandstone

d. chocolate-colored clay shales with fossil leaves; upper part of these are black and below they pass into -

e. whitish clay shales

3. Eocene (36 to 35.5 m.y.)

Wall Mountain Tuff - multiple flow, simple cooling unit of moderate to dense welding with autoxitic foliation; reddish - brown to buff; potassic calc-alkalic rhyolite; glassy sanidine and argillized andesine form principal phenocrysts, with some biotite.

4. Eocene (39 m.y.)

Echo Park Alluvium - rusty red, massive, coarse grained, porphyritic; composed of microcline feldspar and quartz, minor plagioclase feldspar, clusters of sabiotite mica and hornblends. Red color due to finely disseminated hematite.

The geomorphological elements seen today at Florissant are primarily the results of mid-to-late Cenozoic volcanism, uplift, basin-range style of block faulting, and attendant erosion. These events are summarized in the following outline.

1. Laramide Orogeny - 72 m.y. ago; the uplift of the Central Colorado Rockies initiated in the western part of the state elevated the area probably no more than 3000 feet.

2. Eocene Erosion - 50 m.y. ago; paleo valleys indicate drainage was to the east and southeast into the Great Plains. At that time, the Great Plains and the uplifted Rockies were at nearly the same elevation. Many monadnocks were produced along with a nearly flat erosional surface. The products of this erosion filled basins and grabens formed by the Laramide Orogeny. At Florissant, the Echo Park Alluvium was deposited by streams draining down Currant and Oil Creeks.

3. Eocene Volcanism - 36 m.y. ago; massive ashflows emanating from the Mt. Aetna caldera in the Sawatch Range resulted in the formation of the Wall Mountain Tuff. Stream conglomerates containing clasts of the tuff were deposited over the tuff and are found today as the Tallahassee Creek and Castle Rock conglomerates.

4. Eocene Volcanism - 34 m.y. ago; volcanism begins at Thirty-Nine Mile Volcanic Field. Products include: ashflows, lava flows, agglomerates from lahars, breccias, ashfalls, lake formations at Florissant and Antero in South Park. Guffey Volcano and others in the field are formed as composite volcanoes. This indicates a violent type of eruption similar to Mt. St. Helens type.

5. Neogene Uplift and Block Faulting - 25 to 7 m.y. ago; during early Miocene (20-25 m.y.) upper Arkansas River went south into the San Luis Valley. By Pliocene (7 m.y.) drainage became blocked south of Poncha Springs; Arkansan diverted eastward. Divide paleovalley became blocked to the east; drainage diverted to South Platte River and through Florissant. Florissant Basin became filled with alluvial deposits.

6. Late Pliocene - 3 m.y. ago; uplift accelerates, runoff increases with greater elevation of the mountains; the shoulders on Pikes Peak become prominent. Area rises 4,000 ft. (1,200 m.) during this time (Florissant approaches its present 8,400 ft. elevation.

7. Quaternary Glaciation and Alluviation - 2.5 m.y. to present; glaciers form on Pikes peak, but not in Florissant. Climate becomes wetter, colder. Much alluvium and slope wash is produced and deposited in Florissant Basin. About 10,000 years ago, area warms and dries. Grape Creek begins to cut canyon through Pikes Peak Granite to the north of the park. Modern drainage in area becomes established and erosion exposes fossil beds.