Overview of the Geology of the Texas Hill Country

The geology of the Texas Hill Country has been dominated by sedimentation from ancient shallow seas, and igneous uplift events. Glaciers never reached the Hill Country, so natural lakes are not present in the area.

Subject	Topic	Description
Overview	Overview of the Geology of the Texas Hill Country	The geology of the Texas Hill Country has been dominated by sedimentation from ancient shallow seas, and igneous uplift events. Glaciers never reached the Hill Country, so natural lakes are not present in the area.

General Information

The geology of the Texas Hill Country is characterized by sedimentary limestone deposits that were several thousand feet thick. These limestone deposits were formed over millions of years during geologic periods when much of Texas was covered by a shallow, warm, inland sea. The limestone deposits occur in many successive formations, most of which are nearly flat and largely un-deformed.

Karst Topography

The result is a Karst topography that predominates throughout the Hill Country. This karst topography is typified by eroded surface and underlying bedrocks that are quite porous and filled with caves. Karst landforms generally result from mildly acidic water acting on soluble bedrock such as limestone or dolostone. The carbonic acid that causes these features is formed as rain passes through the atmosphere picking up carbon dioxide (CO2), which dissolves in the water. Once the rain reaches the ground, it may pass through soil that may provide further CO2 to form a weak carbonic acid solution.

Considerable faulting occurred over millions of years, particularly in the areas surrounding the Balcones Escarpment, and the remaining fault lines and boundaries between the various limestone strata provide pathways for infusion of rainwater into the underground aquifers. Since rainwater is slightly acidic and the limestone is easily dissolved by acidic solutions, a number of large subsurface caves formed throughout the Hill Country. These caves support extensive aquifers, the most notable of which is the Edwards Aquifer that supplies the water for San Antonio and some of the surrounding communities.

Water flowing underground may form karst rivers, which may alternately flow on the surface and then may disappear underground a number of times and spring up again in different places. This phenomena occurs in the Cibolo Creek just east of Boerne, Texas. An underground karst river also occurs in southeastern Kendall County where the Trinity Aquifer drains directly into the Guadalupe River, below the river’s surface.

Karst topography can create major difficulties for human inhabitants as the result of sinkholes, which develop gradually as surface openings enlarge and form subterranean caves. Progressive erosion of the roof of the underground cavern is not noticed until the roof of the cavern suddenly collapses. Such events have swallowed homes, cattle, cars, and farm machinery. Farmers in karst areas must plan appropriately for the lack of surface water. Soils in these areas may be sufficiently fertile and rainfall may be adequate, but rainwater quickly infiltrates through the soil and crevices in the underlying rock, leaving the surface soil parched between rains.

The Balcones Fault Zone

The Balcones fault zone is an extensive group of faults that transects Texas on a generally southwest to northeast line, and extends into Louisiana and Arkansas. The Balcones fault is marked by the Balcones Escarpment, an abrupt change of elevation in the terrain that extends from Del Rio, through Brackettville, Uvalde, the northern parts of San Antonio, and then northeastward through New Braunfels, San Marcos, Austin, Georgetown, Salado, and Temple. The change in elevation of the terrain is quite abrupt, with the terrain north of the fault ranging from 1,000 to 2,000 feet above sea level.  South of the Balcones Escarpment, the terrain is generally 600 feet above sea level or lower, declining gently for about 150 miles to the coast of the Gulf of Mexico.

The Llano Uplift

In the north-central part of the Hill Country, surrounding the town of Llano, there is a large area of igneous rock called the Llano Uplift. While this area appears to be a granitic intrusion similar to a batholith, it is actually quite ancient. The granite of the Llano Uplift is relative light compared to the underlying rocks in the earth’s mantle, and the crust of the earth is considerably thicker in this region. The result is that the Llano uplift floats on the earth’s mantle, somewhat like a cork floating on water, and it is consequently raised above the height of other rocks in the area.

The oldest rocks in Texas date from the Mesoproterozoic and are about 1,600 million years old. These Precambrian igneous and metamorphic rocks underlie most of the state, and are exposed in three places: (a) the Llano uplift, (b) Van Horn, and the (c) the Franklin Mountains, near El Paso. The rocks in the Llano Uplift were formed approximately 1.35 billion years ago, and were subsequently covered with limestone deposits as much one thousand feet thick. Erosion of the limestone over millions of years caused the underlying igneous rocks of the Llano uplift to be exposed again.  The most dramatic example of the igneous rocks of the Llano Uplift is found at Enchanted Rock in Llano County.

The Llano Estacado

The Llano Estacado ("Staked Plains") of west Texas and eastern New Mexico marks the southernmost extent of the High Plains of North America. It is one of the largest expanses of near-featureless terrain in the U.S. and one of the largest tablelands on the continent - an uplifted surface of porous, uneroded Late Tertiary river sediments veneered by late Pleistocene and Holocene wind-blown sand. The size of the Llano Estacado is about 250 mi. north to south, and 150 mi. east to west, an area of 37,500 square miles. It is a very flat, semiarid plateau, ranging in elevation from 5000' on the northwest to less than 3000' on the southeast, sloping more or less uniformly to the east-southeast at a rate of at least 10' per mile. The slope is imperceptible to an observer on the plateau. The Llano is dry and treeless, with the prevailing wind from the southwest, and mirages are frequent under the hot sun.

The distinguishing characteristic of the Llano Estacado is the Caprock Escarpment, which is seen most prominently on the north and west sides. The escarpment is a precipitous cliff usually about 300' in height that seems to be an almost impenetrable defense for the plain. The cliff on the north facing the Canadian river was seen by Francisco Vásquez de Coronado in 1541 on his way east from Cibola leading him to name the plateau the Llano Estacado, or Palisaded Plain.

The surface of the Llano is remarkably flat, reminding one of the sea, and it is conceivable that the curvature of the earth could be perceived as it is on the sea. The area around Levelland and Brownfield, Texas, is a good place to look for grain elevators sinking beneath the horizon. However, the surface is not uniformly flat, containing shallow draws, often without external drainage. Many temporary, and some permanent, lakes are found, often occupying 'blowout' basins in the loose, dusty surface.

All streams flowing east from the Llano Estacado have their origin on the caprock and flow out onto the lower plains through the Caprock escarpment. That drainage flows eastward out of Palo Duro Canyon and Tule Canyon. Both are tributaries of the Prairie Dog Town Fork of the Red River. Running Water Creek, the Double Mountain Fork, and Yellow House Creek are tributaries of the Brazos River. Sulfur Springs Creek, McKenzie Draw, Seminole Draw, Mustang Creek, Monument Draw, and Johnson Draw are all tributary to the Colorado River. Formations on the east, north, and western boundaries of the Llano Estacado probably account for its name. These steep escarpments of fifty to 300 feet were caused by the slumping of the less resistant beds that underlie the hard, resistant Caprock.

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REFERENCES

· Terry G. Jordan, Handbook of Texas Online, Hill Country, http://www.tsha.utexas.edu/handbook/online/articles/HH/ryh2.html,

· Wikipedia, the free encyclopedia, Texas, http://en.wikipedia.org/wiki/Texas

· Wikipedia, the free encyclopedia, Karst topography, http://en.wikipedia.org/wiki/Karst

· Wikipedia, the free encyclopedia,Image:Texas Hill Country Near I-10, 2004.jpg, http://en.wikipedia.org/wiki/Image:Texas_Hill_Country_Near_I-10%2C_2004.jpg#filehistory

· Wikipedia, the free encyclopedia, List of counties in Texas, http://en.wikipedia.org/wiki/List_of_Texas_counties

Compiled from various sources by

Joe Cooper

Voices of the Texas Hills

Kendall County, Texas

Created: September 14, 2009

Updated: August 22, 2010