WSP06750 - Laserfiche WebLink

Home Browse Search

O)l935 Chapter 3 - Affected E1t~ironnuml The QUaternary geomorpMc history of the Aspen Highlands ridge is primarily responsible for shaping its topographic form, which allows skiing on the broad nose of the ridge, while more vigorous geologic processes dominate the steep ridge flanks. Repeated glacial erosion in the valleys of Castle and Maroon creeks has progressively oversteepened the lower flanks of the ridge. Cirque glaciers eroded large bowls into the ridge in its southem third, and left a legacy of steep, bare bedrock cliffs and ridges. In contrast, the nose of the ridge stood above the glacial ice throughout the Quaternary. and frost wedging and permafrost conditions led to shattering of exposed bedrock. Sheetwash and creep then rell'ansported this weathered rock to form the veneer of colluvium which covers the gently-sloping parts of the ridge nose. As a result of this glacial action, Aspen Highlands now divides two watersheds, Castle Creek and Maroon Creek. GEOLOGIC HAZARDS The geologic hazards that present potential dangers to life or property mapped in Aspen Highlands (Figure 3.2 and Table 3.1), are typical of steep mountain environments. All nine geologic hazards mapped deal with slope instability and its erosional and deposition side effects, and almost all occur on the lower, oversteepened ridge flanks or in the vicinity of the tributary cirques and gullies in the southwest part of the ski area. A summary of the areas and distribution of geologic hazards in the two watersheds is also presented in Table 3.1. UNSTABLE AND PoTENTIALLY UNSTABLE SLOPES Slopes in Aspen Highlands may be unstable as a result of deep-seated bedrock movements, failure of shallow soil layers, or excessive creep or erosion. When ground slope is steep and almost the same in direction and steepness as the dip of sedimentary bedrock, bedrock can slide along weak bedding planes or joints. This condition is restricted to the north facing dip slopes of steep tributary ridges at Aspen Highlands. At several sites, colluviwn at the head of a steep swale has failed downslope as a thin slab of saturated soil material overlying bedrock. These scars represent only a small fraction of the shallow debris slides that have occurred in the ski area. Every drainage basin that ends in an alluvial fan or debris fan has experienced many debris slides. However, since the evidence for shallow debris slides is rapidly obscured by revegetation and eventual covering of scar areas by renewed colluvial deposition in the gully head, Figure 3.2 probably underestimates the occurrence of past debris slides particularly in the steep tributary basins of Maroon Creek. Several small areas were mapped as undergoing excessive surface erosion and rilling. These areas are both on steep, unforested natural slopes, and on maintained ski trails. On the ski lI'ails, erosion seems to be the result of removing forest cover and scraping off topsoil on very steep slopes. This has exposed bedrock slabs in some ski trails, but these are not mapped as erosion areas because future erosion on bedrock will be negligible. compared to erosion rates of unstable, unconsolidated weathered rock and soil (regolith) overlying the bedrock. Slopes with past rock slides and wedge failures possess a high probability for future movement. The next-most- likely sites for slope failure are those areas adjacent to existing slope failures, especially if the adjacent slopes have the same surface slope and bedding angle as the failed area, and they have been undermined by rock masses downslope pulling away from them, creating steep scarps. Potential instability exists for dipslopes adjacent to existing rockslides and wedge failures. particularly if they have the same slope and aspect as the failed slopes. Such areas occur both on the Maroon and Castle creek sides of the ridge (Figure 3.2). Potential instability was also identified in areas which have not failed previously but where geomorphic characteristics suggest incipient slope failure. The potential for catastrophic rockslide failure of areas mapped as potentially unstable is probably low, but is difficult to quantify since the geometry of the deep failure surfaces in bedrock is unknown. During the spring of 1995, heavy snows in April and May, combined with cold temperatures, resulted in up to 300% nonnal snowpack in many areas of Colorado, Suddeo warming in early June led to rapid snowmelt, ground saturation, and several slope failures at Aspen Highlands. Slides occurred near the Moment of Truth Catwalk and Golden Barrel Trail, and severe gullying occurred along the upper part of the Moment of Truth ski run and Geology and Soils Geolog ic Hazards 3-3