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.
Overview of the Geology of the Texas Hill
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.
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
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
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.
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
The Balcones Fault Zone
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
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.
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
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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Terry G. Jordan, Handbook of Texas Online, Hill
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,
Compiled from various sources by
Voices of the Texas Hills
Created: September 14, 2009
Updated: August 22, 2010