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. r " ..: .( '. l ,j - 30 - . Since most of the sediment carried by Indian Wash consists of fine-grained materials (clay, silt, and fine sand), it is believed that almost all of the sediment will be deposited within the reservoir below the emergency spillway crest elevation. In time a certain amount of gravel,.cobbles,and boulders may be deposited at the upper end of the reservoir above spillway crest elevation; but the volume of these materials should be relatively small. Geologic Investigations A prel iminary geologic investigation "las made at the proposed floodwater-retarding structure site Dl-l. Because of the hazard class (UCU) and height of the proposed structure and the presence of founc.ation materials of questionable bearing strength, it was decided that the preliminary investigations at this site should be somewhat more extensive than those usually made during the planning stage. An .attempt was made to obtain the amount and type of geologic data and soil samples that would be needed by the Soil Mechanics Laboratory in making a general evaluation of site conditions as related to cost estimations for the work plan. The investigations carried out at the TIJ-l dam site included geologic mapping, the drilling 'of test holes along the dam centerline and in potential borrow areas, and the collection of both disturbed and undisturbed soil samples. . Twenty test holes were drilled with a power auger along the centerline of the proposed dam in order to obtain infor.mation on the type and extent of the materials underlying the foundation of the dam. All holes were drilled into shale bedrock except where large boulders or cobbles were encountered and the auger bit was unable to penetrate any deeper. ' Bedrock underlying the darn site is the Mancos shale of Cretaceous age. The Mancos in this area consists primarily of a soft to finn, fissile to thin-bedded, silty clay shale. Occasionally very thin beds of hard siltstone and fine-grained sandstone are present. In this area the shale can be placed in four classes or zones, based mainly on the degree of weathering: (1) decomposed shale; (2) weathered shale; (3) partly weathered shale; and (4) unweathered shale. .The valley bottom at the proposed dam site is underlain by alluvial materials with a maximum thickness of about 33 feet. The materials consist mainly of silt and clay mixtures "rlth some fine sand. Thin layers of gravelly well graded sand with some silt and clay also are present. Several representative undisturbed drive samples of the silty clay materials were obtained from the upper layers on both sides of the valley. .