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Rinderle Construction Surficial Geology Investigation, High Desert AFT Subdivision D & 35 Roads, East Orchard Mesa, Palisade, CO May 4, 2001 Page 3 5. LOHMAN, S. W , 1965, Geologic and Ar(esian Wafer Supply, Grand Junction Area, Colorado, Prof. Paper -!5 /, l a9 P 6. SCHWOCHOW, SD., 1978, Mineral Resources Survey ojMesa County, U. S. Colo. Ceol. Survey Resource Series 2. l09 p. Seismic events have occurred near the greater Grand Junction area, but have not been further identified as to source. It is believed the Glade Park Fault, which is west of the Jacobs Ladder Fault Complex and the Ladder Creek Monocline would have some potential for these low intensity earth quakes. These events, within the greater Grand Junction area, were evaluated as having Richter Magnitudes up to and including 4.4, with no reported damages. SITE GEOLOGY Much of the site is underlain by unconsolidated alluvial soils, believed to be 10 to 40 feet thick. A review of Grand Junction Lincoln DeVore's records indicates no exploration borings of our record are within 500 to 1000 feet of this proposed subdivision. A limited gravel survey was conducted on a site across Sink Creek, to the west of this site. As part of this current project, I 1 exploration pits were placed across the north, northwest portion of the tract, Lots I - 9. Coarse grained gravel or cobbles of the Ancient Colorado River deposit were encountered in some of the deeper holes. These coarse grained gravel or cobbles were found to be 12 to 18 feet thick. The surface exposures in this area indicate the Mancos Shale is the bedrock. The surface soils on portions of this site consist of a series of silty clay and clayey silt soils which are a product of an Alluvial fan feature which originates on the West-facing slopes of the Grand Mesa. The Alluvial fan feature is a small part of a very extensive Alluvial fan complex along the base of the Grand Mesa and extending to the Colorado River. The fine grained surface soils are believed to be deposited ovtr the medium dense gravels and cobbles of the Ancient Colorado River deposit, which is in tum deposited over the dense formational material of the Mancos Shale of Cretaceous Age. The Mancos Shale is described as a thin bedded, drab, light to dark gray marine shale, with thinly interbedded fine grain sandstone and siltstone layers. Some portions of the Mancos Shale are bentonitic, and therefore, are highly expansive. The majority of the shale, however, has only a low to moderate expansion potential. GROUND WATER True ground water in this area is normally encountered at significant depths below the ground surface. Some sandstones beds within the Mancos Shale are known to contain ground water but, under most circumstances ground water is contained within the underlying Dakota, Morrison and Wingate Formations. The water-bearing members of these formations are believed to be in excess of 1000 fee[ below the ground surface on this site. In addition, much of the water contained within these formations is mineralized and often saline. Some perched waters are present on this site, along the gullies of Sink Creek and associated tributaries. These perched waters are usually mineralized and contain significant amounts of soluble sulfate salts. Evidence of this perched water was observed on this site during our surface reconnaissance. Due to the proximity of the Mancos Shale Formation, there exists a possibility of a perched water table developing in the alluvial soils which overlie the shale in the areas planned for construction of residential structures. This perched water would probably be the resuh of increased irrigation due to the presence of lawns and landscaping and roof runoff.