Laserfiche WebLink
Climatically, the yearly average temperature is 32 to 45 degrees F., and there is a <br />maximum of 100 frost -free days in a year. Annual precipitation is about 18 to 20 inches <br />(USDA SCS 1976). <br />Paleoenvironmental data for the area is scant, but it is agreed that gross climatic <br />conditions have remained fairly constant over the last 12,000 years. However, changes in <br />effective moisture and cooling/warming trends undoubtedly affected the prehistoric <br />occupation of the area. Prehistoric land use was primarily for hunting and gathering, which <br />had little or no adverse affect on the local environment. Present day land use includes cattle <br />grazing, hunting, and coal mining development. <br />PALEOCLIMATE <br />Based on an analysis of the area's late Quaternary stratigraphy, the geologic history of <br />the last 18,000 calendar years in the region follows something like the following scenario <br />(Miller 2010: Chapter 2; Conner et al. 2011:2.35-2.36). [Dates (*) are calibrated. ] <br />Late Pleistocene dissection scoured channels during the Late Glacial and deposited <br />thick sequences of large, e.g., boulder -sized, gravel in most drainages. About 13,400 BC* <br />the glaciers are retreating and capacity and competence decrease; the time between then and <br />about 11,000 BC* is identified by Haynes (1991) as the Clovis drought. In areas dominated <br />by aeolian processes, deflation occurs. <br />The Younger Dryas, from around 10,600 to 9000 BC*, the last gasp of the glacial <br />period, took place around Folsom times. During the period, drainages are rejuvenated, <br />surfaces stabilize, soil formation accelerates, and the late Pleistocene -early Holocene loess is <br />slowly accumulated. <br />Between 9500 and 5500 BC*, a long drought occurs (interrupted once around 7000 <br />BC*, coincident with Pryor Stemmed occupations). Aeolian sand seas form in Colorado, <br />Wyoming and Nebraska, and drainages throughout the mountain west are choked with <br />sediment and become braided; these are Kaycee equivalent deposits. Dunes form in places in <br />western Colorado and are later preserved as clay dune cores, but Kaycee equivalent deposits <br />varying from a few to several meters in thickness are ubiquitous in the LSFO area. The <br />Pleistocene extinctions were completed early in this interval and Paleoindian big game <br />hunters were subsequently replaced by Archaic hunter -gatherers. While extinction of most of <br />the Pleistocene megafauna took place in Clovis times, mammoth (e.g., Agenbroad 1978), <br />camel and horse persisted in some areas to around 9000 BC* (e.g., Miller and James 1986). <br />Cooling temperatures between 5500 and about 3100 BC* sustained the middle <br />Holocene incision. Capacity and competence increased, but not to the levels achieved during <br />the Late Glacial. As a consequence, when incision exposed Late Glacial gravel, stream <br />3 <br />