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-20- snow observed in the entire study area at the beginning of our survey in • early June and it had disappeared by late June. Bedrock potholes filled by summer rains are another potential source of water that are known to have been used by prehistoric peoples. Our field impressions, however, indicate that large potholes do not tend to form in the sedimentary rocks of the area. Also, those depressions we did observe the day after a severe thunderstorm held little water, probably due to seep- age into the relatively porous sandstone. Precipitation in the area today varies from 10 to 15 inches a year, with considerable local and annual variations. While the picture of past climates in the area is not well known, there is evidence for significant climatic variation during the late Pleistocene and early Quaternary. Even minor long-term variations in climate could significantly alter the hydro- graphic situation existing today, drying up streams that are now permanent or creating permanent watercourses where there are now interroittents. Rich- mond (1965) in a study of glaciation in the Rocky Mountains, outlined a sequence of five glacial episodes during the Pleistocene followed by a warm, dry period termed the "Altithermal Interval", which was in turn succeeded • by a smaller glacial period called the "Neoglaciation". Of particular in- terest here is the Pinedale Glaciation (24,000 to 6,500 B.P.), the Alti- thermal (6,500 to 4000 B.P.) and the Neoglaciation (4000 B.P. to present). While there is some dispute about the precise effects of Rocky Mountain glaciations on the climates of surrounding areas, it can generally be assumed that climatic conditions in the study area would have been somewhat wetter and cooler than at present during the stadials of the Pinedale and hotter and drier than present during the Altithermal. While the hydrological variable is important, even more significant effects would have occurred in the faunal and floral regime of the area, and these changes are of par- ticular significance with respect to prehistoric occupation of the area. 4.2.3 Flora Judging from the known prehistory of the study area and the surrouc~ding region, the prehistoric inhabitants depended entirely on hunting and the exploi*_ation of wild plant foods for subsistence. Consequently, knowledge of the floral regime of the study area is of particular importance in un- derstanding the pattern of prehistoric human occupation. The "sagebrush . steppe" defined by Kuchler (1964) occurs over the greater part of the study