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3.1 III. GEOLOGY AND SOILS 3.1 GeoloQY In the vicinity of the town of Telluride, Colorado, the local geologic section consists of layered Paleozoic and Mesozoic [66.4 to 286 million years (MY) old] sedimentary rock overlain by a thick SE!quence of Neogene (1.6 to 23.7 MY old) volcanic rock. The sedimentary and volcanic rocks are heavily intruded by igneous rocks and faulting is moderate to intense in this area. As shown on Figure 3.1, the town of Telluride is located on an alluvial fan deposit at the mouth of Cornet Creek. An extensive deposit of Pleistocene (0.01 to 1.6 MY old) glacial drift is located directly upstream of this alluvial fan. Smaller deposits are located to the northeast and northwest of the town (Burbank and Luedke, 1966). The glacial drift is composed of rock !,roded from the higher portions of the Cornet Creek watershed, and a reddish soil has developed on the surface of the deposits. Figure 3.2 is a photograph of an exposure of glacial dri ft in the lower part of the Cornet Creek watershed. The gl ac i a 1 depos its consist of unconsol idated and unsorted boulder- to silt-sized fragments in a clay matrix. These deposits are characteriz~j as low-shear strength materials that are prone to failure under reliltiveiy .1 ight load;; (l.uedke and Burbank, 1977). The glacial drift deposit in the lower portion of the Cornet Creek drainage has been mapped as an area of potentially unstable slopes by the USGS (Luedke and Burbank, 1977). The upper peaks at the drainage divide for the watershed were mapped as an area of potential geologic hazard due to rockfall. The geologic materials present in thE! Cornet Creek watershed can be categorized on the basis of deposit type, resistance to erosion, and stability. Figure 3.3 shows a delineation of materials based on these criteria. It is seen from the fi gure that the bedrock depos its in the upper two -th i rds of the watershed are hard and resistant to erosion. The map units that are unstable and least resistant to erosion are units A through C. Unit A consists of talus and rock glacier deposits. The portion of the un it B depos it located just above the tOlin is composed of gl ac i a 1 dri ft and exhibits unstable slopes. The unit B deposit in the upper portion of the watershed is also composed of glacial drift, field evidence indicates that it is relatively stable. The geologic formations that make up unit C are soft sandstones, mudstones, shales, and a bed of fractured limestone. The fractured 1 imestone is underlain by easily eroded sandstone, and is subject to erosion due to this weak underlayer and the high dips present in the Cornet Creek area.