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r . formations were named for landforms within Douglas County. Dawson arkose was named after the Dawson Butte on West Plum Creek, about seven miles west of the project area. The Castle Rock conglomerate was named for the caprock on Gastle Rock, about five miles northwest of the project area. The surface geology of the project area consists of Tertiary-age ' geologic formations. Wall Mountain Tuff, a light gray fine- grained rhyolitic volcanic rock forms a large deposit here, which lies across the western half of the project area (Richardson 1915:Map 30). A narrow finger of rhyolite is exposed in the central area and it outcrops along the banks of Willow Creek. The rhyolite occurs only in the Castle Rock area. It was formed by volcanic activity in the vicinity of the Platte-Arkansas divide (Richardson 1915). An eruption of a small amount of tuff was followed by a flow of rhyolite. The location of the vent and the original extent of the rhyolite flow are not known. The rhyolite ' forms a cap on hills and small mesas between Palmer Lake and Sedalia. Rhyolite forms the caprock on prominent buttes like Raspberry, Larkspur, and Dawson. The rhyolite can occur 30 to 20 feet in depth but is generally a thinner bed. ' Castle Rock Conglomerate occurs across the eastern half of the area (Richardson 1915: Map 30). The Castle Rock Conglomerate ' overlies the Wall Mountain Tuff. The Castle Rock Conglomerate was formed from outwash and fluviate deposits Erom Dawson arkose and other rocks from the Front Range (Trimble and Machette 1979).. It is a well-indurated bouldery cobbly gravel composed mostly of ' Precambrian rocks, but also containing cheat and Tertiary volcanic rocks. The level is generally less than 15 m (50 ft) in thickness. Cherry Creek and its tributaries have cut through ' this formation and the conglomerate forms cliffs in the area and it is the 90-ft caprock on Castle Rock. Dawson arkose, underlying the rhyolite, occurs at the south edge ' of the project area and it lies along the drainages at the northwest edge of the project area and in the Willow Creek drainage (Richardson 1915: Map 30). The Dawson arkose is a group of deposits that vary in texture and color but resemble a feldspar-rich sandstone. The contact of a coarse-grained sandstone member of the Dawson arkose overlying a more compact clayey member results in numerous springs. Petrified wood occurs in both the Dawson arkose and the Castle Rock Conglomerate. Petrified wood and rhyolite were used by prehistoric and historic people in the area. ' Elevation in the project area varies from 6,800 to 6,660 feet. The mesa top in the project area is dominated by a short-grass prairie (Mayberry 1973). The dominant plants are buffalo grass ' (Buchloe dactyloides) and cheat grass (Bromus tectorum) but blue grama (Bouteloua gracilis), xentucky blue grass (POa pratensis, introduced), and western wheat grass (Agropyron smithii) occur in ' the area. There are occasional open stands of scrub oak (Quercus gambelii) and scattered scrub oak, skunkbrush (Rhus trilobata), 8