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Og~S37 Chapler 3. Affecled ElIviro1ll7lt!lU ROCKFALlS RocIcfall hazard areas have been mapped where steep bedrock cliffs occur immediately upslope of talus deposits. No attempt was made to include all areas where solitary rocks might roll or slide down slopes. because this would involve large areas of low to very low hazard. The location of the downslope limit of these hazard areas is approximate, and would vary according to the size of the rockfalls. GRAVlTAnONAL SPREADINS ATypical bedrock ridges and troughs are found in the highest elevations of the ski area, and are inferred to result from slow, deeirseated gravitational spreading (sackung) of large parts of the mountain mass. Troughs resulting from gravitational spreading were mapped south of Loge Peak by McCalpin and Irvine (1995). Such deep-seated movements are expressed by vertical movement along fault lines and by tensional opening of fissures. Loge Peak Area The small isolated bedrock knob, on which the five towers of the original Loge Peak Lift stand. is a natUral feature that is separated from the main summit of Loge Peak by a narrow saddle. This saddle lines up with the linear east face of Loge Peak, parallels the prominent joiot set. and appears to overlie a sackung crack. About 250 feet east of the lift terminal, another isolated hill at the east margin of the ridge crest is likewise separated from the main ridge by a small saddle. probably underlain by a sackung crack. In addition to these inferred sackung cracks, Bryant (1972) mapped a nonheast-lI'ending fault just east of the Loge Peak Lift terminal. Between this fault and the access road, an elongated hot spot (probably due to geothcnnally warmed air that circulates in open fractures and warms the winter snowpack from beneath) may also be related to fracturing of rock due to gravitational spreading. Much of the surface geology in this area is obscured by fill placed during consll'UCtion of the Loge Peak Lift terminal, but evidence from old aerial photographs and geologic maps suggests that the spreading processes are also acting in the area of fill. OlympiC Bowl Area Another gravitational trough was identified in 1995 on the north and east margins of Olympic Bowl. This feature is comprised of a curvilinear 2.400-foot-Iong nonhwest-lI'ending scarp that partly dermes a natural drainage on the southwestern margin of the Upper Robinson's Trail. At the nonhwest end of the scarp is a large natural closed depression that evidently absorbs all the runoff from the drainage. In addition, there are two hot spots along the southwestern margin of the Upper Robinson's Trail. The lower hot spot coincides with a depression known as the Bear's Cave while the upper hot spot was located at a smaller, similar feature, termed the Gopher Den. Besides these hot spots, large open fractures in bedrock were observed during excavation of a waterline in the area of the Ooull N'me patrol cabin. These observations indicate that the 2.400-foot-long scarp, along which the two hot spots and anomalous drainage occur, is a sackung scarp formed by the headwall of the Olympic Bowl pulling away (westward and downslope) from the rest of the ridge. Two westward displacements along the downslope projections of the sackung trough are consistent with the inferred spreading directions and would explain the anomalous scarp higher on the slope (Geo-Haz 1996). SOIL AND TALUS CREEP Since it is slow and restricted to upper parts of colluvial or residual mantles, soil and talus creep are not especially dangerous geologic hazards. They were included on the Figure 3.2 for disclosure only. Surface development is relatively immune to damage by soil creep llecause foundation elements penetrate below the active layer. 011lER HAzARDS Anificial hazards such as upsll'eam dams, underground mines, and large open pits do not occur in the ski area. Aspen Highlands does, however, have moderate earthquake hazards (Kirkham and Rogers 1981). In the period January 1944 to December 1990,63 earthquakes occurred within a 60-rnile radius of the ski area. The largest earthquake was magoitude 5.3 (9/10/69. distance 58 miles). Thiny-eight of the 63 earthquakes (60 percent) occurred in two swarms, the Carbondale swarm of May 3-14,1984 (16 quakes ofmagoilUde 2.1-3.2) and the Geology and Soils Geologic Hazards 3-7