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Aspe. Highlands Ski Area - Draft Enviro1ll7l<NaI/"'f'OL'1 SlalemeN Crested Butte swarm of August 13 to September 22,1986 (22 quakes of magnitude 2.1-3.5). Of the two shon faults mapped in the ski area (near Loge Peak), neither is considered to be active (Kirkham and Rogers 1981). SOILS The distribution of soil map units in the Aspen Highlands Ski Area are shown in Figure 3.3, and their physical and soil management characteristics are presented Table 3.2. The soils units can be broatHy classified in five groupings (Irvine 1993): 1) deep alluvial soils developed on river floodplains, terraces, and on the rolling moraine and outwash landforms at the foot of the ridge (soil units 10-22); 2) soils developed on lateral moraine, alluvial fans, and debris fans (soil units 23-26); 3) soils developed on relatively thick colluvium on the slopes of the ridge crest below the Midway (soil units 33-37); 4) steep forested pans of ridge flanks with soils developed on thin colluvium and residuum (soil units 28, 31, 38-45); and 5) soils developed on residuum on the very steep, rocky. unforested slopes near Highland and Maroon bowls (soil units 29-30, 46-93) plus rock outcrops and rubble land. Each group is described only in general terms in the following five paragraphs and readers are referred to McCalpin (1994) and Geo-Haz (1996) for more detailed descriptions of individual soil characteristics. On floodplains, terraces, and ground moraine, soils occur on 0 to 15 percent slopes, with depth to bedrock greater than 72 inches. Most of these soils have bouldery or stony surfaces, are skeletal, and have cobbly sandy loam textures. Some finer-grained (silt loam) soils also occur in closed depressions, wetlands, and where fine-grained overbank or colluvial sediments accumulate. These soils are relatively stable because of their very gentle slopes, generally coarse texture, and high inftltration capacity. Consequently, erosion is generally not a problem with these soils except along unprotected banks of perennial sll'eams. Revegetation potential is typically high, but can be limited to drought-tolerant species in topographically high areas (due to the excessive drainage of soils), and to riparian plants in areas with a high water table or seasonal flooding. Soils developed on alluvial fans and debris fans occur on 5 to 65 percent slopes. and depth to bedrock is typically greater than 72 inches. Most have bouldery or stony surfaces, are skeletal, and have cobbly sandy loam textures. These soils occur at the sites of frequent deposition from debris flows, debris avalanches, rockfalls, and snow avalanches. Erosion is less of a hazard thac is active deposition, but erosion can occur in debris-flow channels. Revegetation of major shrub and lI'ee species occurs naturally on these slopes between avalanche episodes, but occUrs more rapidly where soils are less stony and have higher organic content. On the relatively broad crest of the ridge from Cloud Nine to the base of the ski area, soils are developed on colluvium and regolith on 5 to 65 percent slopes. Depth to bedrock varies from 12 inches to greater than 72 inches in a complex pattern determined by a buried undulatory bedrock surface beneath the smooth, colluvium- covered slopes. Most soils have stony surfaces, are skeletal, and have a silt loam texture near the surface, rapidly grading downward to cobbly sandy loam. These soils are somewhat prone to erosion and rilling (due to medium to rapid runoff), especially where forest cover has been removed for ski runs on slopes exceeding 40 percent. Considerable erosion has occurred at the outfalls of artificial drainage ditches. Revegetation on the steeper slopes is limited by the high surface erosion rates, so some surface stabilization lI'eatmenl is needed to ensure regrowth. Soils developed on the steep. forested flanks of the ridge are generally developed on slopes of 40 to 65 percent, and typically vary in thickness from 12 inches to 56 inches, depending on local erosional and depositional processes. Soil textures range from gravelly sandy loams (for soils developed on sandstone-derived regolith) to gravelly clay loams (for soils developed on siltstone- or mudstone-derived regolith). Most of these soils, which predominate in most of the active avalanche chutes, display moderate mass movement potential (due to steep slopes and shallow soils) and moderate to high soil erosion hazard (due to medium to rapid runoff on steep slopes). Revegetation potential is limited in most areas by the excessive soil drainage, which leads to seasonal droughty conditions, as well as by recurrent avalanches in chutes. Steep cliffs of exposed bedrock and talus fields dominate much of the Maroon Creek flank of the ridge south of the Midway. Soils developed on these very steep (40 to 150 percent) slopes are generally thin (less than 20 inches), very coarse in texture (cobbly to flaggy sandy loams), and excessively drained. Both soil erosion hazard Geology IJIId Soils 3-8 Soils