Editing Florissant Formation (section)

==Geology==
[[File:Florissant Fossil Bed National Monument, Colorado (9179298199).jpg|thumb|left|2013 aerial photo]]
In the late Eocene to early Oligocene, approximately 34 million years ago, the area was a lake environment with redwood trees. The basement is the [[Proterozoic]] aged Pikes Peak Granite. There is an unconformity from the Pikes Peak Granite to the next unit, the Wall Mountain tuff.<ref name="Foos, 1999">Foos, A. & Hannibal, J. (1999). Geology of the Florissant Fossil Beds National Monument. Field Guide, 9 pages</ref> The massive unconformity is due to erosion that occurred during the uplift of the modern Rocky Mountains, the [[Laramide Orogeny]].<ref name="Steven, 1997">Steven, T. A. (1997).  Middle and Late Cenozoic Tectonic and Geomorphic Development of the Front Range of Colorado. Rocky Mountain Association of Geologists, 1997, Colorado Front Range Guide Book, p. 115-125</ref> The Wall Mountain Tuff was deposited as a result of a large eruption from a distant [[caldera]]. The Florissant Formation itself is composed of alternating units of shale, mudstone, conglomerate, and volcanic deposits. There are six described units within the Florissant Formation. In order from bottom to top: the lower shale unit, lower mudstone unit, middle shale unit, caprock conglomerate unit, upper shale unit, and the upper pumice unit. Each of the shale units represents lacustrine environments, composed of very thin shales that are abundant in fossils, which alternate with tuffs from eruptions.<ref name="Foos, 1999"/> The lower mudstone has been interpreted as a stream environment with the top of the unit being a [[lahar]] deposit. The mudstones were deposited on a valley floor, but not in a lake. The separation of the shale units by non-lake deposits could mean that there were two generations of lake deposits. [[Lahar]]s that went through the valley could have dammed up the valley, allowing for the creation of a lake. The middle and upper shale units were then deposited in this second generation of the lake. The caprock conglomerate was deposited as a large lahar went through the valley and accumulated down on the lake floor.<ref name="book"/>

The Laramide Orogeny, which created the modern Rocky Mountains, had been uplifting the area to the west since the end of the [[Cretaceous]],<ref>Dickinson, W. R. et al (1988). Paleogeographic and Paleotectonic Setting of the Laramide Sedimentary Basins in the Central Rocky Mountain Region, Geological Society of America Bulletin, v. 100, p. 1023-1039</ref> although the exact timing of the orogeny is debated <ref name = "Steven, 1997" /> In the late Eocene to the Early Oligocene, volcanic episodes began to occur to the southwest of the Florissant area. These episodes of eruption would deposit ash and other volcanic debris on the Florissant location, and the volcanic material would be one of the most important factors in the fossilization of the plants and animals that are so abundant in the formation. The fossil bearing paper shales are intercalated with larger deposits of volcanic material.<ref name="book"/><ref name="Harding,2000">Harding, I. C., & Chant, L. S. (2000). Self-Sedimented Diatom Mats as Agents of Exceptional Fossil Preservation in the Oligocene Florissant Lake Beds, Colorado, United States. Geology (Boulder), 28(3), 195-198.</ref>

Most of the rocks that were deposited after the Oligocene and before the Pleistocene have been eroded away.<ref>Chapin, C. E., & Cather, S. M. (1983). Eocene Tectonics and Sedimentation in the Colorado Plateau, Rocky mountain Association of Geologists, p. 33-54</ref><ref name="Gregory, 1992">Gregory, K. M., & Chase, C. G. (1992). Tectonic significance of paleobotanically estimated climate and altitude of the late eocene erosion surface, Colorado. Geology (Boulder), 20(7), 581-585</ref> Most of the remaining units are composed of clasts of weathered Pikes Peak Granite, volcanics, and mud that were transported by streams that flowed through the area. Some mammoth bones have been found within these units and have been dated to around 50,000 years old.<ref name="book"/>

===Thirtynine Mile Volcanic Field===
[[File:Big Stump.jpg|thumb|upright|right|A large petrified stump of ''[[Sequoia affinis]]'' at Florissant Fossil Beds National Monument<ref name = "website">Florissant Fossil Beds. 2012. National Park Service. 5 November 2012. http://www.nps.gov/flfo/index.htm</ref>]]
Around 25–30 kilometers to the southwest, a series of stratovolcanoes, similar to modern day volcanoes like Mt. St Helens, developed and erupted periodically. Called the [[Guffey, Colorado|Guffey]] volcanic center, within the larger [[Thirtynine Mile volcanic field]], the volcano had eruptions that included domes, lava flows, and pyroclastic events.<ref>Chapin, C. E., & Epis, R. C. (1964). Some Stratigraphic and Structural Features of the Thirtynine Mile Volcanic Field, Central Colorado. The Mountain Geologist, V. 1, p. 145-159</ref><ref>Wobus, R. A. et al. (1990). Geochemistry of high-potassium rocks from the mid-Tertiary Guffey volcanic center, Thirtynine Mile volcanic field, central Colorado, Geology, V. 18, No 7, p. 642-445</ref> Ash from these events settled throughout the area and lahars flowed down the valleys. The ash that settled created the tuff, and the lahars formed the mudstones and the conglomerates of the Florissant formation.

The Florissant was deposited in a paleovalley after one of the lahars dammed it up. The resulting lake became as large as 36&nbsp;km<sup>2</sup>.<ref name = "Veach, 2008" /> There were two cycles of lake environments. The first one created the lower shale unit, while the second lake created the middle and upper shale units.<ref name="book"/>
Eventually, the volcanoes became dormant and started to erode away. Over time, the volcanoes became so eroded that there are no more obvious signs of them on the surface. Instead, the Eocene erosional surface is the only remnant of the area's volcanoes.<ref name = "Gregory, 1992" />