The Devonian-Mississippian transition outcrops of central Texas are here described summarily and assigned to a new stratigraphic unit, the Houy Formation. The beds included are mainly Upper Devonian, but partly Lower Mississippian. Locally a basal fraction may be Middle Devonian. Although the deposits included are diverse and their associations complex, the maximum surface thickness so far known is only about 17 feet.

Pleistocene-age fossil horses of Texas belong to at least three faunal assemblages. The oldest contains Hippotigris (= Plesippus) which disappeared before Yarmouthian time. The second contains Equus scotti and Onager semiplicatus (= Equus (Asinus) calobatus) which disappeared before Sangamon time. Both these groups are found on the High Plains but not on the Coastal Plain or along the central Texas rivers, where a later fauna, probably of Sangamon age, is found which contains remains of Equus, Asinus, and Onager but no Hippotigris.

Fall Prong quadrangle is in the marginal portion of the Edwards Plateau near the southeastern corner of the Llano region. Almost three-quarters of the quadrangle is within the Edwards Plateau, and the rest consists of valleys of the Llano basin cutting back into the plateau.