[In preparation]
[Note the county lines. Perhaps add the county names.]
Abbott & Woodruff, Preface to “The Balcones Escarpment – Geology, Hydrology, Ecology and Social Development in Central Texas”, 1986*
[In preparation]
The Balcones Escarpment is a line of low hills that extends through Central Texas. As pointed by Fenneman in his classic Physiography of Western United States, this escarpment marks the of break between two grand physiographic Province divisions of North America: the Great Plains Province on the west and the Coastal Plains on the east. In Central Texas this major physiographic break is denoted by the change from Hill Country/Edwards Plateau uplands on the escarpment to the Blackland Prairie on the east. The Balcones Escarpment lies along the major line of dislocation of the Balcones fault zone, a series of en echelon mainly down-to-the-coast formal faults. Fault displacements have resulted in Lower Cretaceous limestones to the west being juxtaposed against Upper Cretaceous claystones, chalks, and marls to the east.
The Balcones fault zone is a surface expression of a deep-seated crustal discontinuity. The Ouachita orogeny extends southward from the Arbuckle/Ouachita juncture in Oklahoma beneath Central Texas to the Rio Grande where the orogen is displaced laterally into Trans-Pecos Texas. The Ouachita complex forms a hinge between the stable continental interior and the still-subsiding Gulf Coast Basin. Balcones faulting was probably a result of periodic adjustments across this buried hinge.
The main tectonic events of Balcones faulting are generally thought to have occurred during the Miocene, but there is considerable evidence that periodic structural adjustments also took place in the Cretaceous. For example, mafic alkalic volcanic plugs of Late Cretaceous age occur at the surface and in the subsurface all along the Balcones fault zone. The geochemistry and petrology of these igneous rocks suggest that they penetrated the entire crust of the Earth.
Dramatic changes in the landscape occur across this crustal discontinuity. On the west are plateau uplands and ruggedly dissected limestone hills; soils are thin and stony, and the main agricultural use of the land is for range; the dominant native vegetation assemblage is juniper-live oak savannah; and groundwater is generally of good quality with ample quantities occurring at shallow depths from limestone aquifers. On the east side of the escarpment, terrain consists of rolling prairies and broad river bottom;; soils are thick fertile clays, which compose prime arable land; native prairie grasses were dominant before the agricultural activities supplanted them; and groundwater in large amounts is available only at considerable depth and is commonly tepid and brackish.
The escarpment is also a major weather-maker. Maximum relief is only a few hundred feet, but it is the first topographic break inland from the Gulf of Mexico and thus acts as an orographic influence on unstable, water-laden air masses. These combined factors have resulted in the Balcones Escarpment being the locus of the largest flood-producing storms in the conterminous United States. The greatest single rainfall event ever recorded in the contiguous U.S. occurred in 1921, when 38 inches of rain fell in 24 hours near Thrall in Williamson County.
Physiographic changes have had their impact on human culture as well. As pointed out by Peter T. Flawn in his article “The Everlasting Land” (in A President’s Country, Shoal Creek Press, 1964), the Balcones Escarpment marks the line where the American West really begins. It marks the boundary between the cotton economy of the Old South and the cattle economy of the Old West. This geocultural break has been the site of many major towns and cities in Texas: Del Rio, San Antonio, Austin, Temple, Waco, and (in its broader geographic context) Dallas/Fort Worth and Sherman/Denison. The cities are a response to the geologic break in the same way that geologic changes along the Fall Line of the eastern United States created favorable sites for industry and commerce. People settled along the Balcones Escarpment in order to draw on both economies – cotton and cattle. Also, their endeavors were aided by the dependable water supplies provided by the great springs that issue forth along the fault zone.
A dramatic expression of the Balcones/Ouachita discontinuity is the localization of the Edwards aquifer. The Edwards aquifer iscomposed of karstified groundwater reservoirs that extend along the Balcones Escarpment in Central Texas and west beneath the vast Edwards Plateau. The Edwards Limestone, the main host rock of the aquifer, once extended unbroken across much of Texas – from the middle part of the Gulf Coastal Plain to the Panhandle and Trans-Pecos regions. This oncecontinuous rock unit has been flexed and broken by tectonic events, dissected by erosion, and dissolved by the actions of percolating water. The vagaries of erosion and of structural deformation have resulted in different hydrodynamic regimes within different geologic/geographic provinces. In areas where the Edwards Limestone crops out, groundwater occurs under water-table conditions. Downdip within the fault zone, a confined zone of groundwater occurs. Farther downdip, water quality changes abruptly across a “bad-water line” that marks the edge of upwelling deep-basin waters. Locally within the Gulf Coast basin, the Edwards Limestone is a reservoir rock for hydrocarbons.
The Balcones fault zone is where the Edwards aquifer is most prolific. There, faults have provided major avenues for directional porosity and permeability, and these conduits have been enlarged by solution so that great volumes of water flow rapidly from west to east along the escarpment. In this way, the semi-arid western part of the fault zone provides water for the sub-humid eastern watersheds where the largest springs in Texas occur.
*Abbott, P., and C. Woodruff, 1986. The Balcones Escarpment – Geology, Hydrology, Ecology and Social Development in Central Texas. Published for the Geological Society of America Annual Meeting, San Antonio, Texas. November. 212 p.