Fossil remains from alluvial deposits of the central Texas rivers and Coastal Plain constitute a single faunal unit, indicating that current concepts regarding development of paired rock-cut river-terrace sequences require revision. Tectonic and/or eustatic changes cannot account for paired river terraces because of the local nature of tectonic activity and remoteness from the sea. The cause must be found in a mechanism widely applicable. Evidence recounted here indicates that this mechanism is climatic control exercised through intervention of plant cover. With increased humidity, plant cover is increased, direct runoff and soil wash are greatly decreased, and streams are perennial and entrench themselves. Under arid conditions plant cover is sparse, runoff and soil wash are vastly increased, streams are intermittent, entrenchment is nil, and valley walls retreat laterally, forming paired terrace surfaces by processes of pedimentation. With multiple terrace steps, a terrace surface formed by escarpment r...