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• The material in the alluvial fan is an unsorted and unstratified <br /> mixture of all sizes from silt and clay up to 7-foot boulders. <br /> The high fraction of very large rock sizes and the large amount <br /> of fines (silt and clay) indicate that the material was <br /> transported in a very high energy, debris-choked stream that <br /> could move giant boulders. The gradient of the present Rapid <br /> Creek is about 600 feet per mile, as the drainage descends from <br /> its head at 10,000-foot elevation on Grand Mesa down to its <br /> confluence with the Colorado River at about elevation 4, 800 <br /> feet. The large volume of stream flow that would have been <br /> necessary to move such a volume of material is believed to have <br /> resulted from the melting of an ice cap on Grand Mesa in <br /> Pleistocene times. The debris-laden stream moved down the steep <br /> gradient of Rapid Creek and deposited its load as an alluvial <br /> fan as the stream gradient lessened near the Colorado River. <br /> The fan material exposed by excavation at the proposed quarry <br /> site is an unsorted mixture of approximately 50 percent passing <br /> the No. 4 screen size (approximately J inch) and 50 percent <br /> that would be retained on a No. 4 screen. The coarser fraction <br /> contains all sizes of gravel, cobbles, and boulders but seems <br /> to consist of a large amount of 6-inch to 36-inch rocks with <br /> some boulders up to 7 feet across. The rocks are almost entirely <br /> of hard, dense basalt which has originated from the lava capping <br /> on Grand Mesa. The fragments range from angular to subrounded <br /> • and are generally unweathered with varying amounts of vesicles <br /> (gas holes) . <br /> A typical profile of the material at the quarry site consists <br /> of 6 inches of dark brown top soil, 12 inches of clayey soil <br /> with some cobbles, and the underlying fan deposit of very coarse <br /> material. The near-surface soils have been mapped by the Soil <br /> Conservation Service as Utaline stony loam (UOD) . <br /> The exact depth of the alluvial fan has not been determined, <br /> but the landowner has excavated a pit to a depth of 35 feet <br /> without reaching bedrock. By projecting from exposures on the <br /> nearby canyon, it appears that the deposit could be as thick <br /> as 50 feet. Using a conservative depth of 30 feet as an overall <br /> average thickness of deposit, and an arbitrary area of 7 acres <br /> (leaving about 2 acres for storage and a buffer around the <br /> property perimeter) , the quarry site could yield up to 335, 000 <br /> cubic yards of total materials. The excavated materials would <br /> obviously need processing to produce whatever the market demand <br /> might be at the time. With size separation, the quarry could <br /> produce a high quality riprap for erosion protection, as well <br /> as other size ranges that could be desirable for "road subgrade <br /> or surfacing, miscellaneous fill at construction sites, or <br /> landscaping stone. <br /> A ground water table within the alluvial fan materials is not <br /> • expected due to the topographic relief of the proposed quarry. <br /> The ground elevation on the property is about 160 feet above <br /> the nearby Rapid Creek. <br /> 2 <br />