The Yucaipa 7.5' quadrangle is located at the southeastern margin of the San Bernardino Basin, an extensional region situated within a right-step-over zone between the San Jacinto and San Andreas Fault zones. The quadrangle is traversed by several faults of the San Andreas system, including (from oldest to youngest) the Banning Fault and the Wilson Creek, Mission Creek, Mill Creek, and San Bernardino Strands of the San Andreas Fault. The Mill Creek Strand of the San Andreas Fault is the easternmost strand of the San Andreas in the Yucaipa quadrangle. It separates granitic and metamorphic rocks of the San Bernardino Mountains block from a thin slice of similar rocks on Yucaipa Ridge, and thus has only a small amount of strike-slip displacement. The Wilson Creek Strand traverses Yucaipa Ridge and converges toward the Mlll Creek Strand in the Santa Ana river Canyon. The fault has juxtaposed an igneous and metamorphic complex (Wilson Creek block) and overlying nonmarine sedimentary rocks (Mill Creek Formation of Gibson, 1971) against rocks of San Bernardino Mountains-type, and thus has significant strike-slip displacement. The Mission Creek Strand is inferred to lie beneath Quaternary surficial deposits along the southwestern base of the San Bernardino Mountains. This fault is the major strand of the San Andreas Fault zone, and has juxtaposed crystalline rocks of San Gabriel Mountains-type (including Pelona Schist overlain by the Vincent Thrust and associated upper-plate crystalline rocks) against the Wilson Creek block and the San Bernardino Mountains. The San Bernardino Strand defines the modern trace of the San Andreas Fault. The strand forms primary fault features in all but the youngest Quaternary surficial units, and is thought to have evolved in the last 125,000 years or so based on regional fault relations. Complications within the San Andreas Fault system over the last several hundred thousand years have created a landscape setting in which Quaternary surficial materials of the Yucaipa quadrangle have accumulated. Crustal extension throughout the San Bernardino Basin region led to uplift of the Crafton Hills block and down-dropping of the Yucaipa Valley region on faults of the Crafton Hills and Chicken Hill complex. Subsequent middle and late Quaternary streamflows deposited several generations of axial-valley and alluvial-fan sediment in the down-dropped lowlands. These deposits and the older San Timoteo beds they overlie record the history of Quaternary fault movements, and form reservoirs for ground water in the Yucaipa quadrangle. Digital Data: The geologic database of the Yucaipa 1:24,000-scale 7.5' quadrangle, San Bernardino and Riverside Counties, California, was prepared by the Southern California Areal Mapping Project (SCAMP), a regional geologic-mapping project sponsored jointly by the U.S. Geological Survey and the California Geological Survey. The database was created in ARC/INFO (Environmental Systems Research Institute, ESRI), and includes the following files: (1) a readme.txt file, (2) this metadata file, (3) coverages containing geologic data and station-location data, (4) associated INFO attribute data files, (5) a browse graphic (.pdf) of the geologic-map plot and map-marginal explanatory information, (6) a PostScript graphics file of the geologic-map plot with map-marginal explanatory information, and (7) .pdf text files describing the map units of the Yucaipa quadrangle (Description of Map Units) and their geologic age and correlation (Correlation of Map Units).