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~:%'~~• sufficient time for the growth and maturation of domesticated crops such a maize. The elevation of the project area varies from 6,900 feet (2,103 m) to about 7,800 feet (2,377 m) above mean sea level, over 1,000 feet (300 m) higher than Trinidad. Because of these elevational differences, climatic condition in the upper Purgatoire Valley ~= differ somewhat from Trinidad. Generally, these higher elevations are cooler, wetter, and have a shorter frost-free period (Lutz and Hunt 1972:2). Summer rainstorms occur with f~ remarkable regularity and provide much of the annual precipitation; this rainfall is augmented by winter snowfall. ~ Temperatures are also variable, as described below. Temperatures range from -29 to +32° C (-20 to +90° F)during the year, with the warmest months (July and August) averaging 22° C (71 F) and the coldest months (December and January) averaging -5° C (23° F). During the summer, fluctuations of 22 C (40° F) in daily temperature are not unusual. The Picketwire Valley has afrost-free period of 130-150 days (Gleichman 1983:6) {': ' Geomoroholovv The project area is located in the Park Plateau subdivision of the Raton section, Great Plains physiographic province (Fenneman 1931). In general, this section is characterized by high mesas, extensive dissected plateaus, deep canyons, and volcanic mountains of various ages (Fenneman 1931:37). The Park Plateau L straddles the Colorado-New Mexico border, extending 60 miles northward to Huerfano Park and 30 miles southward to Ute Park on the upper Cimarron River (Fenneman 1931:40). It is widest at the state boundary, extending approximately 4o miles from the Sangre •`'- de Christo Range to Raton Mesa. The Plateau is underlain by the Raton formation, composed of late Cretaceous through early Tertiary sandstone, siltstone, and coal-bearing shale (Gleichman 1983:5). The Purgatoire River is the major drainage in the Colorado portion of the Park Plateau. The river heads in the Culebra Range as three separate drainages" the North, Middle and South Forks. The North and Middle Forks come together at Vigil, near the western edge of the project area, and the South Fork joins the main watercourse at Weston. The Purgatoire then flows eastward within a relatively wide valley (locally referred to as the Picketwire Valley) for approximately 30-35 miles. At Trinidad, it flows to the northeast for a short distance, turns once again to the northeast, an orientation it maintains until it joins the Arkansas River at Las Animas. The total length of i the Purgatoire River is approximately 150 miles. The project area is drained by a small number of small intermittent drainages, most of which flow south or east into the Purqatoire.