This report summarizes results of geologic mapping in the South Umpqua Falls region of the Western Cascade Range in Douglas County, southwestern Oregon (plate 1). Rocks of the Western Cascades range in age from late Eocene to late Miocene and consist of deformed and partially altered flows, pyroclastic rocks, and interlayered sedimentary rocks (Peck and others, 1964). In the area of this investigation, Tertiary units include the Colestin Formation of late Eocene age, the Little Butte Volcanic Series of Oligocene and early Miocene age, and the Sardine Formation of possible middle and late Miocene age. It is notable that older Tertiary rocks, present both to the north and to the south, are absent beneath the rocks of the Colestin Formation in this region. In the vicinity of Tiller, west of the mapped area, pre-Tertiary plutonic and metamorphic rocks form the basement and are overlain with profound unconformity by strata of the Colestin Formation. Rocks of the Western Cascade Range are capped to the east by younger volcanic rocks of the High Cascade Range.