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-3- <br />a working ranch. Most of the second and third terraces aze irrigated pasture and alfalfa <br />fields with some non-irrigated range land in the northwestern part of the property. <br />GEOLOGIC SETTING <br />The second and third alluvial terraces on the property aze underlain by glacial <br />outwash gavel deposited by the Eagle River during the late Pleistocene between about 12 <br />to 35 thousand years ago. Formation rock in the area is the Eagle Valley Evaporite that <br />crops out at several places along the lower part of the escarpment between the first terrace <br />and the second and third terraces. Colluvium covers the formation rock between outcrops <br />on the escarpment. Major faults have not been mapped in the area (Tweto and Others, <br />1978). <br />Because of the numerous cobbles and boulders in the alluvial terrace deposits it <br />was not possible to drill auger borings through the alluvium into the underlying Eagle <br />Valley Evaporite. Practical auger refusal was encountered at depths between about 4 and <br />16 feet below the ground surface. Soil profiles encountered in the auger borings <br />consisted of an upper sandy clay and silty sand from 0.5 to 8.0 feet thick overlying <br />outwash gavel. A 0.5 to 1.0 foot thick organic enriched topsoil is present at the ground <br />surface. The outwash gravel consists of rounded, stratified gravel, cobbles, and boulders <br />in a silty sand matrix. Along the terrace escarpment to the north of the proposed gravel <br />pit it appears that the Eagle Valley Evaporite is present i^ about the lower two-thirds of <br />the escarpment and that the outwash gravels are present in the upper one-third. This <br />indicates that the outwash gravels in the northern part of the gravel pit area aze about 20 <br />feet thick in some places. <br />The Eagle Valley Evaporite is made up of gray and tan gypsum and anhydrite <br />with interbedded siltstone, claystone, shale, and dolomite. Bedded to massive gypsum <br />deposits outcrop along the terrace escarpment. The gypsum and anhydrite are soluble i^ <br />fresh water. The siltstone, claystone and shales vary from non-cemented but firm to <br />cemented and hard. The dolomite is cemented and hard. The bedding structure is <br />convoluted because of flow deformation in the plastic gypsum and anhydrite. Subsurface <br />H-P GEOTECH <br />