This report is designed to provide geologic data for planning the use of mineral resources (including ground water), the disposal of domestic and industrial wastes, and foundation engineering for structures in Allen County. The surficial morphology of the county, its linear moraine ridges, glacial and postglacial drainage routes, and lake plain, reflects various modes of glacial and glacial lake or stream deposition. The near-surface unconsolidated materials, clayey glacial till, stream-deposited sands and gravels, and lake clay, reflect these origins as well. But beneath these surface materials lies buried a stack of similar deposits, including hard loamy tills, of earlier glacial episodes. The surface of the buried hard till mirrors the present surface topography, much of which may be inherited from that earlier glaciation. Drift ranges from less than 40 feet to more than 300 feet in thickness, and it covers an erosional surface developed on Devonian and Silurian limestone and dolomite and on Mississippian black shale. The predominantly limestone and dolomite bedrock in Allen County was deposited in an ancient sea basin centered in Michigan. The rock is tilted slightly northward. The basin margin environment proved conducive to reef growth at times, especially during the Silurian Period.