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<br />- 25 - <br /> <br />above and to the north of the apex of the major mudf10w fan, (figures <br /> <br />1, 10, 12, 13, 14, 15, 16). The cliffs are composed of strongly <br />faulted, jointed, and fractured Cretaceous rocks. The lower part is <br /> <br />shale and sandy shale of the Mancos formation, and the upper part con- <br /> <br />sists of alternating shale and sandstone beds of the Mesa Verde forma- <br /> <br />tion. The rapid rate of weathering, erosion, and back-wasting has pre- <br /> <br />vented development of vegetative cover, and provides superabundant <br /> <br />debris for the Slate Creek mudflows. Gallo Bluff is entirely within <br /> <br />the watershed of Slate Creek. Erosion of the cliffs is so active that <br /> <br />one can see and hear minor rockfalls constantly while in the vicinity <br /> <br />of the bluff. Several factors conspire to produce the rapid erosion <br /> <br />of the cliffs. Both sandstone and shale are strongly jointed, (figures <br /> <br />13, 14, 15, 16, 17), including vertical joints which are both parallel <br />and at approximate right angles to the retreating c1iff1ine. In addi- <br />tion, the shale air-slakes rapidly, providing an easily activated clay <br /> <br />matrix for mudflows. Numerous spring seeps between sandstone and shale <br /> <br /> <br />units saturate and soften the weathered shale and rock debris (figure 12). <br /> <br /> <br />The weathered shale is removed by minor sloughing and sliding as well as <br /> <br /> <br />by sheet and rill wash. <br />The above described processes produces a general "Grand Canyon" <br />type of topography with waste slopes of shaley debris alternating with <br />nearly vertical ledges of broken sandy shale or sandstone (figure 13). <br /> <br />The average slope of the highest part of the cliff is approximately 36 <br /> <br />degrees, or 75%. At this location, the bluff has 1,400 ft. of vertical <br />rise in 1,900 ft. horizontal distance. <br />The more resistant sandy layers frequently are undercut by the <br /> <br />rapidly wasting shale beds, and the sandstone ledges in many cases con- <br />