The Rim Rock country, on the western edge of a vast Trans-Pecos Texas lava field, received thousands of feet of volcanic material during the Tertiary Period. Although pyroclastic rocks predominate, several flow rocks form the most distinctive stratigraphic markers. Dikes, sills, and laccoliths were emplaced following the cessation of extrusive igneous activity. The Vieja Group, comprising the oldest volcanic strata, has been tentatively assigned to the Chadronian Stage (Lower Oligocene). Late Tertiary block-faulting created an intermontane area which subsequently received thick bolson deposits. Recent climatic fluctuations have resulted in the development of gravel-capped terraces along the Rio Grande.