The "case history" of Folsom Dam emphasizes the need for geologic reasoning in the site location, planning-design, and construction phases of multi-purpose projects. Construction afforded an unusual opportunity to evaluate current exploration techniques in highly fractured and deeply weathered granitic and metamorphic rocks.The project totals 4.8 miles of dam and consists of: an overpour gravity section with partially enveloping earthfill wing dams on the American River; Mormon Island earthfill dam across an ancestral channel; eight smaller earthfill saddle dams on tributary drainages; two diversion tunnels; and appurtenant structures.The main dam and most of the saddle embankments are founded on quartz diorite (Upper Jurassic). Metamorphic rocks of the Amador group (Middle-Upper Jurassic) form the foundation at the Mormon Island site. In addition to weathered bedrock, overburden consists of Recent mining debris and Pliocene (?)-Pleistocene channel gravels. The quartz diorite is extensively fractured by a complicated joint system; locally the closely spaced joints simulate a shear or fracture zone.The erratic weathering of quartz diorite constitutes a serious foundation and excavation problem. Commonly outcrops proved to be residual "boulders" underlain by highly weathered rock to considerable depths. The composition and origin of minerals in the fresh and weathered stages (slight, moderate, highly) of quartz diorite are tabulated for use in predicting the conditions at new sites involving similar rock; megascopic and microscopic criteria for subdividing the weathered rock are correlated with their respective engineering-physical properties. The most important factors controlling the depth and extent of weathering are: spacing of joints and fractures; degree of microfracturing; and the physiographic history of the area.Geologic investigations included a variety of subsurface techniques, i.e., borings, "man-sized" openings, geophysical surveys, and use of a bore hole camera. Several limitations became apparent with conventional exploration techniques; data from "man-sized" openings supplemented the cored borings.At the main dam, foundation excavation was performed in three contract stages; conditions exposed at subsequent levels aided in estimating the final foundation. The extremely varied excavation and final foundation rock are recorded in an exploded stage diagram; actual subsurface geologic features of typical "lift" surfaces are correlated with logs of cored borings and resistivity survey data. Three sizeable fault zones traverse the foundation; special dental treatment and added excavation exposed unquestionable foundation rock.The core trench of many earthfill dams was grouted on the split-method pattern. Adequate grouting of the highly weathered quartz diorite (moderately permeable) proved difficult due to its inhomogeneity and clayey-filled fractures. At Mormon Island, innumerable springs issued from the schistose foundation and necessitated a grout curtain to a depth of 60 feet.Sources of construction materials consisted of: (1) natural aggregate and pervious fill from alluvium deposits; (2) impervious earthfill "borrowed" selectively from outcrops of highly weathered quartz diorite; and (3) rip-rap of quartz diorite from the main dam, power house and tailrace excavations.To gain additional power head, the natural channel was lowered and the power house placed in a deep excavation. Geologic problems inherent to the excavation included design of a 250 foot open-cut slope and treatment of fractures and joints in a rock "ledge" between the spillway channel and partially underground power plant.