Florissant Formation

The name Florissant comes from the French word for flowering. In the late 19th century tourist and excavators came to this location to observe the wildlife and collect samples for collections and study. The Petrified Forest, which is one of the main attractions at the monument today, lost much of its mass due to collectors removing large amounts of petrified wood from the site. During the 1860s and 1870s the area was mapped by geologists for the first time. Geologists of the Hayden Survey visited the area in the early 1870s, and fossil plants from the beds were described by Leo Lesquereux, fossil insects by Samuel Hubbard Scudder, and vertebrate fossils by Edward Drinker Cope. The formation was first formally named as the Florissant Lake Beds by Charles Whitman Cross in 1894. In 1969, the Florissant Fossil Bed National Monument was established after a long legal battle between local land owners and the federal government. Today, the park receives approximately 60,000 visitors a year, and is the site of ongoing paleontological investigations. The formation itself was renamed the Florissant Formation in 2001 to conform with the requirements of the North American Commission on Stratigraphic Nomenclature. ==Geology==

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. The massive unconformity is due to erosion that occurred during the uplift of the modern Rocky Mountains, the Laramide Orogeny. 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. although the exact timing of the orogeny is debated Most of the rocks that were deposited after the Oligocene and before the Pleistocene have been eroded away. 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. 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 volcanic center, within the larger Thirtynine Mile volcanic field, the volcano had eruptions that included domes, lava flows, and pyroclastic events. 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 km2. 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. 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. ==Fossils==

'' flower These trees could have been as tall as until they were killed by lahars suffocating the oxygen supply to their roots. Dendrochronological examination of the tree rings has resulted in estimated ages of 500–700 years old when the trees were killed and buried. Some of the stumps belong to angiosperms. The insects that are found in the ash-clay beds are diverse and numerous. Mayflies, dragonflies, damselflies, grasshoppers, crickets, katydids, cockroaches, termites, earwigs, web-spinners, cicadas, snake flies, lacewings, beetles, flies, mosquitoes, butterflies, moths, wasps, bees, ants, and other insects have all been found in the Florissant. Of these, the beetles are the most diverse and dominate in number of specimens found as fossils. About 38% of the specimens found in the fossil beds are varieties of beetle. These fossils are aquatic and terrestrial insects, giving insight into the ecology and biology of the area. Surprisingly, no reptiles or amphibians have been recorded from Florissant, where they would have been expected. No explanation for the lack has been proposed, given the nearly 40,000 specimens from this location held by museums. The toxicity of the water due to volcanic activity could be to blame, but there are documented cases of aquatic animals in the lake. ==Paleoclimate==

Fossil plants, and in particular their leaves, have been the most useful sources of information of paleoclimate during the time of deposition of the Florissant Formation. Plants have a smaller tolerance on average to climatic changes, whereas many animals can be mobile and respond to rapid seasonal or daily changes. Comparing fossil plants and leaves to modern analogs enables inferences about the climate to be made based on physiological and morphological similarities. One of the best ways to do this is by studying the characteristics of the leaves that have been found. By looking at the physiognomy, or analysis of gross appearance based on climatic factors, the mean average temperature (or MAT) has been estimated to be around 13 °C, much warmer than the modern MAT at Florissant of 4 °C. There have also been estimates that the MAT was between 16 and 18 °C, based on comparisons to the closest living relatives of the plants. There are also indications that the seasonal changes in the area were not as great as what is seen in modern times. Estimates of MAT, based on pollen, have put the temperature as high as 17.5 °C, but pollen is arguably less diagnostic than leaves. . ==See also==