ELECTRICAL METHODS 



565 



tion of fault zones or other structural conditions that would influence 

 the location or construction of the proposed dam; (2) the thickness of 

 overburden and fill materials overlying the shale bedrock at the dam site 

 and the thickness of fill materials in the topographic depressions along the 

 reservoir rims; (3) depth to and subsurface contour of the top of the 

 Bearpaw shale (bedrock) at the dam site and along the reservoir rims; 

 and (4) depths to and elevations of ground water levels in the proposed 

 dam abutments and in the reservoir rims. 



A generalized stratigraphic section of the area is shown in the 

 right-hand portion of Figure 349. The surface is usually covered with 

 clay which is oftentimes sandy. Beneath this clay layer is found glacial 

 drift containing small or large boulders in a matrix of varying propor- 

 tions of sand, clay, and small gravel. Underneath the glacial drift is a 

 layer of clay, also often of a sandy nature. Beneath this clay layer there 



Fig. 349. — Geophysically-indicated structure and moisture table, and general stratigraphic section of 

 west abutment of Fort Peck Dam. 



occurs a weathered shale layer which is impregnated with water having 

 a high percentage of dissolved minerals. The weathered shale layer is of 

 relatively small thickness. As the depth increases, the weathered layer 

 gradually merges into firm shale bedrock. The weathered shale and the 

 firm shale layers constitute the upper portion of the Bearpaw shale. For 

 purposes of dam site construction, the geophysical interpretation was 

 directed toward determining the bottom of the weathered shale layer, 

 rather than the theoretical "top" of the Bearpaw formation. 



The typical stratigraphic section just described varies to a certain 

 extent throughout the area, and in some cases there is a fairly distinct 

 contact between the glacial drift and the shale formations, with no inter- 

 vening clay, sand, or appreciable weathered shale formation. 



Typical results of the geophysical studies conducted on the west abut- 

 ment are shown graphically in Figure 349 and the survey may be sum- 

 marized as follows : 



