2018-01-03_PERMIT FILE - C1981010 (6)

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Late Pleistocene dissection scoured channels during the Late Glacial and deposited thick sequences of large, e.g., boulder -sized, gravel in most drainages. About 13,400 BC* the glaciers are retreating and capacity and competence decrease; the time between then and about 11,000 BC* is identified by Haynes (1991) as the Clovis drought. In areas dominated by aeolian processes, deflation occurs. The Younger Dryas, from around 10,600 to 9000 BC*, the last gasp of the glacial period, took place around Folsom times. During the period, drainages are rejuvenated, surfaces stabilize, soil formation accelerates, and the late Pleistocene -early Holocene loess is slowly accumulated. Between 9500 and 5500 BC*, a long drought occurs (interrupted once around 7000 BC*, coincident with Pryor Stemmed occupations). Aeolian sand seas form in Colorado, Wyoming and Nebraska, and drainages throughout the mountain west are choked with sediment and become braided; these are Kaycee equivalent deposits. Dunes form in places in western Colorado and are later preserved as clay dune cores, but Kaycee equivalent deposits varying from a few to several meters in thickness are ubiquitous in the LSFO area. The Pleistocene extinctions were completed early in this interval and Paleoindian big game hunters were subsequently replaced by Archaic hunter -gatherers. While extinction of most of the Pleistocene megafauna took place in Clovis times, mammoth (e.g., Agenbroad 1978), camel and horse persisted in some areas to around 9000 BC* (e.g., Miller and James 1986). Cooling temperatures between 5500 and about 3100 BC* sustained the middle Holocene incision. Capacity and competence increased, but not to the levels achieved during the Late Glacial. As a consequence, when incision exposed Late Glacial gravel, stream power was insufficient to erode the gravel and most drainages initiated a cycle of channel widening. Away from drainages, the middle Holocene loess accumulated. Pithouses were in wide use in the Rocky Mountains, Wyoming Basin, and Colorado Plateau in the interval, suggesting more sedentary populations; Yarmony Site in Eagle County and site 5ME16789 near Battlement Mesa are local examples. McKean Complex is well represented in western Colorado during the latter part of the interval and the period of transition to warmer climates that followed. After about 3100 BC*, warming temperatures led to erosion of the loess by 2500 to 1850 BC* and the deposition of the middle Holocene alluvium. Droughts in the late Holocene are best dated by periods of erosion, i.e., lacunas, identified by unconformities in loess deposits. Erosion in loess took place between 1850 and 950 BC*, 275 BC* and 165 AD*, and 1050 and 1350 AD*, and again in the last 150 years or so. The first interval coincides with the Middle to Late Archaic transition and the third interval coincides with the Medieval Warming Period in Europe. In the alluvial system, deposition of the middle alluvium ended after the first and interval, by 650 BC*. The first of Lightning equivalent alluvium is deposited during the second and interval, at some time after 650 BC*. As the suggested dates imply, the two deposits are nearly continuous and appear 3