Precambrian rocks exposed along the river below the dam were intruded, deformed, and recrystallized deep within continental crust between 1850 and 1800 mya, then eroded by several miles before sands were deposited on them during Cambrian time about 500 million years ago. The oldest rocks here are dark gray amphibolites composed of black amphibole and gray plagioclase feldspar, cut by masses of pinkish gray granite made of creamy feldspars and gray, glassy quartz, with unassimilated pieces of amphibolite floating in the granite to form intrusion breccia. Younger granites cut older ones, and some granites are coarse-textured pegmatites with large crystals of pinkish feldspar and quartz. The rocks display distinct layering or foliation produced by solid-state flowage of crushed debris under extremely high shearing stress and or recrystallization under compression, and in places this foliation has been folded, showing an initial high-temperature, high-pressure deformation followed by later deformation. A large amphibolite xenolith in coarse granite contains folded granite stringers similar to the enclosing granite, suggesting deformation of the fragment after intrusion of some of the granite, while elsewhere veined Precambrian amphibolite xenoliths are cut by granite showing foliation, and laminated dark granite with much disseminated amphibolite includes granular fabric produced by intense shearing along a west-northwest trending fault zone after granite intrusion at 1800 mya. Granite dikes and stringers also cut highly deformed and altered amphibolite, including two ages of granite dikes, and the multiple deformations seen here are typical of Precambrian rocks.