SEISMIC METHODS 919 



At the dam site, the Sheep Mountain formation is an andesite or andesite- 

 latite flow which overHes andesite-basalt breccias and agglomerates of the 

 Conejos formation. The andesite flows dip 10 to 15 degrees to the east, 

 with occasional irregularities where the dip is to the southeast or to the 

 northeast. 



These two formations both form steep cliffs which extend to the flood 

 plane of the river in some cases. In places the lower sections are covered 

 by talus material and by alluvial fans. The Rio Grande River has eroded 

 through the flows and breccias to a depth of about 150 feet. The valley 

 floor is generally covered with 3 to 6 feet of loam, but numerous gravel 

 deposits are visible. Beneath the topsoil, the valley fill is made up of well- 

 rounded, moderately clean gravels, sand, and cobbles separated by lenses of 

 clay. 



Reservoir Area. — The reservoir area is enclosed by andesites and 

 breccias which in general dip away from the reservoir basin. The andesites 

 are fairly dense and show not more than the usual amount of jointing 

 common for such rocks. The breccias are often interbedded with andesite 

 flows and show little jointing or fracturing. Seepage losses should be 

 small, as the andesites and breccias are usually well blanketed. 



Left Abutment (Looking Downstream). — The left abutment is a 

 steep cliff, the lower portion composed of a hard, coarse, tight breccia. 

 The upper section, which is probably above the maximum flow line, is 

 fairly dense, hard andesite. Both types of rock should be adequate for the 

 dam proposed. 



Right Abutment. — The right abutment is also a steep cliff of 

 andesite which overlies a section of breccias. The latter are exposed on the 

 west side of the ridge which forms the abutment. The andesite probably 

 extends ± 50 feet beneath the flood plane of the river to the north. Beyond 

 this point the abutment rock is breccia. Both of these rock types should 

 prove sufficiently tight and stable. 



Foundation Area. — The foundation area is a breccia, or andesite- 

 latite, or possibly a combination of both. It is overlain by about 150 feet 

 of sand gravels, cobbles and clay. The foundation rock is tight and stable 

 enough for a dam. The water table is very near the water surface of the 

 river at the dam site. The seismic wave velocity in the saturated overburden 

 was 6,900 feet per second, which is higher than usual for such material. 

 This may indicate that some of the overburden may be stable enough to be 

 left in place. It appears to be sufficiently free-draining so that at least that 

 portion of it at the downstream toe of the dam would not have to be 

 removed. Drilling would be necessary to establish the presence or absence 

 of clay or ash lenses, and the stability of the overburden. 



