An investigation of the mineralogical properties and mechanical composition of soils, river sands, alluvium, and rocks gives detailed information concerning the materials involved in erosion and river development. Samples were collected from several localities in the Middle River basin selected to provide information on the rocks being eroded, the soils being formed, and the alluvium being deposited. The mineralogy of the following formations is described: Chemung formation and Brallier shale of Late Devonian age; Martinsburg shale, Beekmantown dolomite, and Lenoir limestone of Ordovician age; Conococheague limestone and Elbrook dolomite of Cambrian age. The Chemung, Martinsburg, Conococheaque, and part of the Elbrook contain distinctive, though restricted, insoluble residues, and particularly distinctive heavy minerals. Variations in content of quartz, feldspar, and clay minerals are important. The mineralogy of the sands from river beds varies according to the rocks through which the river flows; for example, when the stream cuts through the Martinsburg shale very small zircon grains are added to the bed load; euhedral quartz crystals are added from the Beekmantown dolomite. Detailed descriptions are given of a soil profile developed on the Lenoir limestone and on the Brallier shale. Mechanical analyses of soils and alluvium give the size distribution of material that is being eroded. Residual soils have a high silt (50 2 micron grain diameter) content but very little sand. Coarse chert accumulates in the soil above cherty limestones and dolomites. These investigations were undertaken to provide background information for geomorphological studies being undertaken by John T. Hack jointly with the writer. The clays present in rocks, alluvium, and soils will be described in another paper by John C. Hathaway.