Crater Lake caldera collapsed about 6,850 yr B.P. during the climactic eruption of Mount Mazama, a High Cascade basaltic andesitic to dacitic volcanic center that was constructed during a period of about 400,000 yr. The caldera and the products of the climactic eruption are clear evidence for the presence of a shallow magma body that must have supported a hydrothermal system in the recent past. The geology of Mount Mazama has been mapped at a scale of 1:24,000 based on detailed study of the walls of Crater Lake caldera and mapping of the flanks of the volcano. The map shows lavas and fragmental deposits of Mount Mazama, lavas of nearby monogenetic volcanoes, pre-Mazama silicic volcanic rocks, products of the climactic eruption, and glacial deposits. Related topical studies of the volcanology, geochronology, petrology, and geochemistry of the Crater Lake area depend on field relations established by geologic mapping.