THE ANCESTRAL MEDINA RIVER AND ALLUVIAL SURFACES ADJACENT TO THE BALCONES ESCARPMENT, SOUTH TEXAS

At the foot of the Balcones Escarpment in South Texas braided streams deposited extensive gravels (Uvalde Gravel). The gravels derived from Lower Cretaceous limestones and flints in the Edwards Plateau. The time of deposition is unknown. (For over 100 years it has been called Quaternary? or Pliocene?) The Ancestral Medina River was one of these streams. After the pluvial episodes, during which the Uvalde Gravel formed, that same stream incised about 200 feet and then alluviated about half of the vertical distance. Starting at that level the Ancestral Medina River and its tributaries built a broad, composite alluvial surface (Hondo Surface). Today an underfit stream occupies the southwestward-trending valley of the Ancestral Medina River because a south- and southeastern-flowing stream captured the flow of that river. This piracy produced the modern Medina River.

Another alluvial surface (San Geronimo Surface) developed sloping to the southwest and graded to the modern Medina River when it was at a higher elevation than at present. That surface developed by action of San Geronimo Creek and its tributaries. That same creek impinges against a still higher southeastward-sloping composite alluvial surface (Culebra Surface) built by Culebra Creek and some low-order, interbasinal streams. In developing their surfaces San Geronimo and Culebra Creeks partially removed and covered part of the Uvalde Gravel near the Balcones Escarpment. The order of the surfaces in decreasing age is Uvalde, Hondo, Culebra, and San Geronimo.

Within the modern Medina River valley several paired- and non-paired terraces developed – one older than the San Geronimo Surface and the rest younger. A low, broad terrace (Applewhite Terrace) is recognizable to the confluence of the Medina and San Antonio Rivers. The Medina River incised about 30 feet into this surface. Narrow bands of modern alluvium form the present floodplain.