The Texas Hill Country is a vernacular term that describes a distinct area of Texas, with a distinct culture and heritage. Although the exact boundaries of the Texas Hill Country are not formally defined, it is generally thought to consist of all or part of twenty-seven counties in central Texas.
Geography
The “Texas Hill Country” is not a precise geographic area but a vernacular term that describes an area of central Texas, which includes twenty-seven counties, and is characterized by eroded limestone hills, clear rivers, dense vegetation, and pure air. Generally, the Texas Hill Country is defined by the Balcones Escarpment, which forms its southeastern border, the relatively un-dissected Edwards Plateau, which forms its western boundary, and rolling plains and prairies which form its northern boundary.
The counties included in the Texas Hill Country, include the following:
- Bandera County
- Blanco County
- Burnet County
- Comal County
- Crockett County
- Edwards County
- Gillespie County
- Hays County
- Irion County
- Kendall County
- Kerr County
- Kimble County
- Kinney County
- Lampasas County
- Llano County
- Mason County
- McCulloch County
- Medina County
- Menard County
- Reagan County
- Real County
- San Saba County
- Schleicher County
- Sutton County
- Travis County
- Uvalde County
- Williamson County
Geology
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 Bracketville, 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 underly 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.
Weather
The Texas Hill Country is a transition zone between humid and semiarid climates, which receives widely varying amounts of rainfall. As an example of these variations, Fredericksburg, in the center of the Hill Country, received only eleven inches of precipitation in 1956, but forty-one inches of precipitation in 1957. These variations between flood and drought conditions are explained partly by the cyclical “El Nino” and “La Nina” in the eastern Pacific Ocean which control the position of the jet stream over Central Texas and the inflow of moisture across Mexico. A second contributing factor is the presence of the Balcones Escarpment which causes rapid uplifting of moist air flowing northward from the Gulf of Mexico. These two factors cause the Hill Country to experience periodic and quite severe flooding events.
Water
Underlying the Texas Hill Country there are extensive aquifers, which are cut and exposed by faulting along the Balcones escarpment. Abundant springs also occur at higher elevations where erosion of the limestone has exposed the water table. These springs feed the many rivers in the Texas Hill Country which include the following:.
- Llano River
- Colorado River
- Perdenales River
- Guadalupe River
- Blanco
- San Marcos River
- San Antonio River
- Medina River
- Frio River
- Nueces River
Early settlements were established where these rivers intersect the Balcones Escarpment, because these locations provided abundant water near to good agricultural land.
Vegetation
The vegetation of the Hill Country was originally relatively sparse and park-like. A photograph taken from the top of Bandera County Courthouse in the 1890s showed relatively open grassland prairie with only a few scattered Live Oaks. At that time the Ashe-juniper (commonly called Cedar) was confined mostly to the deep canyons. However, with settlement came new efforts to control the environment, and with those efforts unanticipated consequences.
Before European settlers came to the Hill Country, wildfires were relatively common. These fires often resulted from lightning strikes and occasionally were deliberately set by the Indians to concentrate herds of buffalo and other animals. The wildfires were events that were completely beyond the control of the Indians, and they eventually burned themselves out. The Indians simply moved out of the way of the wildfires, until they passed through the area. Although an inconvenience, the wildfires posed no particular problem or threat to nomadic tribes. However, the European settlers took a distinctly different view of wildfires. Tied to fixed locations, with considerable investment in houses, barns, and livestock, the settlers, took aggressive measures to prevent and suppress fires. The unintended consequence was a total change in the environment and the vegetative ecosystems.
Live Oaks generally survive pretty well through periodic wildfires, but young cedars are totally destroyed by them. That is why the cedars in the Hill Country landscape were generally limited to the canyons and the creek and river bottoms where the effects of wildfires were relatively limited. But when the settlers suppressed the periodic fires, the areas infested with cedars expanded rapidly. Cedars propagate by seeds, contained in “cedar berries,” which are a great favorite of many species of birds. The birds consume enormous quantities of cedar berries, and the seeds pass, unharmed, through the bird’s digestive systems to be planted “pre-fertilized” in the bird droppings. Within a generation of the suppression of wildfires, cedars invaded most of the Hill Country and made much of the land unusable. A mature cedar will consume as much as forty gallons of soil moisture per day and the shade of the cedar trees is so dense that nothing will grow under them.