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2004 Geologic Hazard Field Observations <br />South of Divide Mining Area <br />10.3 Rockfalls <br />Rockfalls are the free falling movement of rocks, which have become detached from cliffs or <br />other steep slopes, and move under the influence of gravity and the underlying ground surface. <br />The detached rocks roll or bounce downhill, depending on the configuration of the slope. Their <br />movement continues downslope until they are stopped by an obstmction or lose potential energy <br />and stop naturally. The rockfall potential increases as the slope increases, particularly where <br />resistant bedrock, such as hard sandstones, is exposed as cliffs or ledges. <br />The steeper slope areas in much of the South of Divide mining area range from about 30 to 55 <br />percent along the Dry Fork of Minnesota Creek and its tributaries, and also in local areas along <br />the main fork of Lick Creek. However, slopes steeper that these values occur in much of the <br />eastern part of the South of Divide mining area. Cliffs and underlying slopes ranging from about <br />40 percent to as much as 80 percent occur along the southward-trending cliffs east of the main <br />fork of Minnesota Creek. However, the steepest, lower cliffs and over steepened slopes east of <br />the main fork of Minnesota Creek are located below the outcrop of the E Seam, and would not be <br />affected by mining (Figure 9 and Map 1). <br /> <br />The rockfall potential is estimated to be low to moderate where slopes underlain by resistant <br />bedrock range from 30 to about 40 percent. A moderate to high rockfall potential is estimated <br />for slopes ranging from about 40 to 80 percent. The risk to people is a function of location. For <br />example, the rockfall potential is very high, and risk to people is very high along Interstate 70 in <br />the canyon west of GIenwood Springs (where a large boulder bazely missed the author in 2001). <br />However, where the rockfall potential is high, such as below the west-facing cliffs east of <br />Minnesota Creek, the risk to people is low, because the area is remote and the access road is <br />traveled infrequently. <br />The areas with steep slopes in the South of Divide mining area, which have the greater potential <br />for rockfalls, are located in either areas with only local access roads, which have only limited <br />travel, or are in areas remote from any access roads or other man-made features. Based on a <br />review of aerial photographs and analysis of the USGS 7.5-minute quadrangles maps, there are <br />631-032.621 Wright Water Engineers, Inc. Page 43 <br />November 2004 <br />