Tuff, tuffaceous sand and clay, bentonite, and sandstone containing abundant volcanic rock detritus are present in Gulf Coast Tertiary rocks ranging in age from Eocene to Pliocene. This report summarizes the results of a stratigraphic and sedimentologic study of outcrops of one unit of the sequence, the Gueydan (Catahoula) Formation, from south of the Colorado River, Texas, to the Rio Grande. It is one phase of a study of middle Tertiary volcanism in southern Texas and northeastern Mexico. The long-range purposes of this project are (1) to determine the petrogenesis and provenance of Gulf Coast Tertiary sedimentary rocks rich in volcanic rock detritus, and (2) to map, describe, and interpret previously unstudied Tertiary volcanic and hypabyssal rocks in northeastern Mexico.