Grand Coulee Dam, under construction on the Columbia River in north-central Washington, is located at the boundary between the essentially horizontal basalt flows of the Columbia Plateau and the complex granitic mountains to the north.
The foundation and abutments of the dam, exposed by excavation of glacial and fluvial overburden, are composed of two types of granite. The older, a coarse-grained, massive, biotite granite, is a marginal facies of the Colville granite batholith. The younger, a fine-grained, slightly porphyritic granite, occurs in vertical dikes, which strike approximately north across the older granite, and represents a resurgent phase of the Colville magma. The rock of the west abutment and western half of the foundation is largely fine-grained granite. The rock of the eastern half and the east abutment is predominantly coarse grained. Irregularly-shaped acid and basic masses are interpreted as differentiates of the coarse-grained granite in which they occur although some may be highly metamorphosed xenoliths.
Three major systems of joints cut the foundation granites. One vertical system strikes NNE, and another essentially vertical system strikes WNW. The third system is essentially horizontal. The joints of the two vertical systems are locally abnormally close-spaced and cause a sheeted structure along narrow, discontinuous zones. These vertical sheeted zones acted to concentrate the circulation of heated solutions, and the hydrothermally altered rock is confined chiefly to them. The solutions were somewhat alkaline and probably preceded and accompanied the extrusion of the earliest plateau basalts.