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~ -4- - ~~ _° 'J • within the escarpment separating the two basins, whereas relatively thin ~ s (6 to 12 feet) colluvlal soils were encountered In the remaining exploratory 7 7 holes. In some of these exploratory holes, from 1~ to 6 feet of clayey :i gravel colluvlum was also encountered between the overlying sandy clay - ~ , colluvium and the underlying bedrock. Colluvial soils within escarpment ~ 0 areas apparently were derived from the erosion and redepositlon of upsiope .~ terrace deposits and bedrock. , ` ~; -~ Bedrock encountered across the site consists of brown claystone and ~ dark gray. fossillferous marine shales of the Cretaceous-age Lewis Shale. 'd The claystone bedrock is weakly cemented and highly Jointed, but well ;! Consolidated. The shale bedrock Ts also weakly cemented and welt eon- '' solidated, but does not exhibit extensive Jointing. Crystallized gypsum i - , •s • was observed on some of the joints of the claystone. Structurally, this silo Is on the southwest limb of the Hayden Syncline near the axis of the ~ fold. Regional mapping Indicates that the bedrock dip is about 5 degrees 1 toward the north. Shale partings observed in the_core also indicate a vary shallow dip. ~ SUBSOIL CONDITIONS The subsoil conditions were Investigated by drilling 10 test holes at the designated locations shown on Flg. 2. Depth of drilling was dntermTned based on proposed excavation and fill depths and subsoil conditions as they developed. Tha graphical logs of exploratory holes are presented on Flgs. 4 and 5, and Figs. 6 and ]show detailed logs of Test Holes 4 and'8, which were cored. Cross sections depicting existing ground and proposed float grade levels and wrresponding subsurface material types and free water conditions are shown on Flgs. 8 through ii.