|The foundation of Mountain Park Dam consists of Cambrian granite which has been intruded by diabase. The granite is hard, fine to medium rystalline, pink to grayish pink, and weathered to a depth of about 2 inches along joints near the surface. It is intricately cut by joints, shear, and faults.|
The canyon in which the dam is constructed is fault and joint controlled. A fault zone approximately 62 feet wide is present in the canyon floor and lies beneath the central portion of the dam. This zone is characterized by sheared, pulverized, granular, slickensided diabase with some granite between the diabase intrusions. The granite in this zone is hard to soft and easily broken. The zone, known as the plug area, was over excavated to a depth of 25 feet, backfilled with concrete, and then grouted. Other areas of soft rock in the abutments and foundation area were also over excavated to a minimum depth of 5 feet, then backfilled with concrete. The foundation area and abutments were grouted before placing concrete.