The Balcones Fault or Balcones Fault Zone is a tensional structural systemin the U.S. state of Texas that runs approximately from the southwest part of the state near Del Rio to the north central region near Dallas along Interstate 35. The Balcones Fault zone is made up of many smaller features, including normal faults, grabens, and horsts. One of the obvious features is the Mount Bonnell Fault.
A tensile structure is a construction of elements carrying only tension and no compression or bending. The term tensile should not be confused with tensegrity, which is a structural form with both tension and compression elements. Tensile structures are the most common type of thin-shell structures.
In the United States, a state is a constituent political entity, of which there are currently 50. Bound together in a political union, each state holds governmental jurisdiction over a separate and defined geographic territory and shares its sovereignty with the federal government. Due to this shared sovereignty, Americans are citizens both of the federal republic and of the state in which they reside. State citizenship and residency are flexible, and no government approval is required to move between states, except for persons restricted by certain types of court orders. Four states use the term commonwealth rather than state in their full official names.
Texas is the second largest state in the United States by both area and population. Geographically located in the South Central region of the country, Texas shares borders with the U.S. states of Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the west, and the Mexican states of Chihuahua, Coahuila, Nuevo León, and Tamaulipas to the southwest, while the Gulf of Mexico is to the southeast.
The location of the fault zone may be related to the Ouachita Mountains, formed 300 million years ago during a continental collision. Although long since eroded away in Texas, the roots of these ancient mountains still exist, buried beneath thousands of feet of sediment. These buried Ouachita Mountainsmay still be an area of weakness that becomes a preferred site for faulting when stress exists in the Earth's crust.
The Ouachita Mountains, simply referred to as the Ouachitas, are a mountain range in western Arkansas and southeastern Oklahoma. They are formed by a thick succession of highly deformed Paleozoic strata constituting the Ouachita Fold and Thrust Belt, one of the important orogenic belts of North America. The Ouachitas continue in the subsurface to the southeast where they make a poorly understood connection with the Appalachians and to the southwest where they join with the Marathon area of West Texas. Together with the Ozark Plateaus, the Ouachitas form the U.S. Interior Highlands. The highest natural point is Mount Magazine at 2,753 feet.
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation and if buried, may eventually become sandstone and siltstone.
In geology, the crust is the outermost solid shell of a rocky planet, dwarf planet, or natural satellite. It is usually distinguished from the underlying mantle by its chemical makeup; however, in the case of icy satellites, it may be distinguished based on its phase.
The Balcones Fault has remained inactive for nearly 15 million years, with the last activity being during the Miocene epoch. This activity was related to subsidence of the Texas Coastal Plain, most likely from the large amount of sediment deposited on it by Texas rivers. The Balcones Fault is in one of the lowest-risk zones for earthquakes in the United States.
The Miocene is the first geological epoch of the Neogene Period and extends from about(Ma). The Miocene was named by Charles Lyell; its name comes from the Greek words μείων and καινός and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene is preceded by the Oligocene and is followed by the Pliocene.
Subsidence is the sudden sinking or gradual downward settling of the ground's surface with little or no horizontal motion. The definition of subsidence is not restricted by the rate, magnitude, or area involved in the downward movement. It may be caused by natural processes or by human activities. The former include various karst phenomena, thawing of permafrost, consolidation, oxidation of organic soils, slow crustal warping, normal faulting, caldera subsidence, or withdrawal of fluid lava from beneath a solid crust. The human activities include sub-surface mining or extraction of underground fluids, e. g. petroleum, natural gas, or groundwater. Ground subsidence is of global concern to geologists, geotechnical engineers, surveyors, engineers, urban planners, landowners, and the public in general.
An earthquake is the shaking of the surface of the Earth, resulting from the sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in size from those that are so weak that they cannot be felt to those violent enough to toss people around and destroy whole cities. The seismicity, or seismic activity, of an area is the frequency, type and size of earthquakes experienced over a period of time. The word tremor is also used for non-earthquake seismic rumbling.
The surface expression of the fault is the Balcones Escarpment,which forms the eastern boundary of the Texas Hill Country and the western boundary of the Texas Coastal Plain and consists of cliffs and cliff-like structures. Subterranean features such as Wonder Cave and numerous other smaller caves are found along the fault zone.
An escarpment (scarp) is a steep slope or long cliff that forms as an effect of faulting or erosion and separates two relatively level areas having differing elevations. Usually scarp are used interchangeably with escarpment.
The Texas Hill Country is a geographic region located in the Edwards Plateau at the crossroads of West Texas, Central Texas, and South Texas. Given its location, climate, terrain, and vegetation, the Hill Country can be considered the border between the American Southwest and Southeast.
Wonder Cave is a show cave located in the Balcones Fault in San Marcos, Texas. Its entrance is one mile southwest of the county courthouse in San Marcos. The cave is reported to be the only commercially operated dry-formed cave in the United States. The fault itself is visible in the cave's ceiling. Originally named Bevers Cave after Mark Bevers, who discovered it in 1893, the cave for a while concealed Bevers' illicit distilling and gambling enterprises.
Many cities are located along this fault zone. Springs such as San Pedro Springs, Comal Springs, San Marcos Springs, Barton Springs and Salado Springs are found in the fault zone and provide a source of fresh water and a place for human settlement.
San Pedro Springs is the name of a cluster of springs in Bexar County, Texas, U.S.A. These springs provide water for San Pedro Creek, which flows into the San Antonio River. The San Antonio Springs also feed into the San Antonio River.
Comal Springs are the largest concentration of naturally occurring freshwater springs in Texas. They are located in the city of New Braunfels and are the result of water percolating through the Edwards Aquifer formation.
San Marcos Springs is the second largest natural cluster of springs in Texas. The springs are located in the city of San Marcos, Texas, about 30 miles (48 km) southwest of Austin and 46 miles (74 km) northeast of San Antonio.
The Balcones Fault Zoneis a demarcation line for certain ecological systems and species distributions, e.g., the California fan palm ( Washingtonia filifera ) is the only species of palm tree that is native to the continental United States west of the Balcones Fault.
Washingtonia filifera, also known as desert fan palm, California fan palm or California palm, is a flowering plant in the palm family (Arecaceae) native to the far southwestern United States and Baja California. Growing to 15–20 m (49–66 ft) tall by 3–6 m (10–20 ft) broad, it is an evergreen monocot with a tree-like growth habit. It has a sturdy, columnar trunk and waxy, fan-shaped (palmate) leaves.
The Arecaceae are a botanical family of perennial plants. Their growth form can be climbers, shrubs, trees and stemless plants, all commonly known as palms. Those having a tree form are colloquially called palm trees. They are flowering plants, a family in the monocot order Arecales. Currently 181 genera with around 2600 species are known, most of them restricted to tropical and subtropical climates. Most palms are distinguished by their large, compound, evergreen leaves, known as fronds, arranged at the top of an unbranched stem. However, palms exhibit an enormous diversity in physical characteristics and inhabit nearly every type of habitat within their range, from rainforests to deserts.
Barton Springs is a set of four natural water springs located at Barton Creek on the grounds of Zilker Park in Austin, Texas, resulting from water flowing through the Edwards Aquifer. The largest spring, Main Barton Spring supplies water to Barton Springs Pool, a popular recreational destination in Austin. The smaller springs are located nearby, two with man-made structures built to contain and direct their flow. The springs are the only known habitat of the Barton Springs Salamander, an endangered species.
The Edwards Plateau is a region of west-central Texas which is bounded by the Balcones Fault to the south and east, the Llano Uplift and the Llano Estacado to the north, and the Pecos River and Chihuahuan Desert to the west. San Angelo, Austin, San Antonio and Del Rio roughly outline the area. The eastern portion of the plateau is known as the Texas Hill Country.
Mount Bonnell, also known as Covert Park, is a prominent point alongside the Lake Austin portion of the Colorado River in Austin, Texas. It has been a popular tourist destination since the 1850s. The mount provides a vista for viewing the city of Austin, Lake Austin, and the surrounding hills. It was designated a Recorded Texas Historic Landmark in 1969, bearing Marker number 6473, and was listed on the National Register of Historic Places in 2015.
Medina Valley, also known as the Medina River Valley, is an area in south central Texas containing the Medina River, Lake Medina, and the Medina Dam. It serves to drain the Balcones Escarpment of the Texas Hill Country and irrigate the farms contained therein. It is approximately 120 miles long and is named for the Medina River.
The Medina River is located in south central Texas, United States, in the Medina Valley. It was also known as the Rio Mariano, Rio San Jose, or Rio de Bagres. Its source is in springs in the Edwards Plateau in northwest Bandera County, Texas and merges with the San Antonio River in southern Bexar County, Texas, for a course of 120 miles. It contains the Medina Dam in NE Medina County, Texas which restrains Lake Medina. Much of its course is owned and operated by the Bexar-Medina-Atascosa Water District to provide irrigation services to farmers and ranches.
The Edwards Aquifer is one of the most prolific artesian aquifers in the world. Located on the eastern edge of the Edwards Plateau in the U.S. state of Texas, it is the source of drinking water for two million people, and is the primary water supply for agriculture and industry in the aquifer’s region. In addition, the Edwards Aquifer feeds the Comal and San Marcos springs, provides springflow for recreational and downstream uses in the Nueces, San Antonio, Guadalupe, and San Marcos river basins, and is home to several unique and endangered species.
Texas contains a great variety of geologic settings. The state's stratigraphy has been largely influenced by marine transgressive-regressive cycles during the Phanerozoic, with a lesser but still significant contribution from late Cenozoic tectonic activity, as well as the remnants of a Paleozoic mountain range.
The Texas Coastal Bend, or just the Coastal Bend, is a geographical region in the US state of Texas. The name refers to the area being a curve along the coast of the Gulf of Mexico. The largest city of the Coastal Bend is Corpus Christi. It includes part of Laguna Madre and North Padre Island, as well as Mustang Island.
San Antonio Springs is a cluster of springs in Bexar County, Texas. These springs provide a large portion of the water for the San Antonio River, which flows from San Antonio to the Gulf of Mexico. The San Pedro Springs also feed into the San Antonio River.
The geography of Texas is diverse and large. Occupying about 7% of the total water and land area of the U.S., it is the second largest state after Alaska, and is the southernmost part of the Great Plains, which end in the south against the folded Sierra Madre Oriental of Mexico. Texas is in the south-central part of the United States of America, and is considered to form part of the U.S. South and also part of the U.S. Southwest.
Salado Springs is the name of five groups of springs at the town of Salado in Bell County, Texas, in the United States. The springs are located 48 miles (77 km) north of Austin or 135 miles (217 km) south of Dallas.
The Mopah Range is a desert mountain range, in the Lower Colorado River Valley region, in southeastern San Bernardino County, California.
Balcones Canyonlands is a national wildlife refuge located in the Texas Hill Country to the northwest of Austin. The refuge was formed in 1992 to conserve habitat for two endangered songbirds including the golden-cheeked warbler and the black-capped vireo and to preserve Texas Hill Country habitat for numerous other wildlife species. The refuge augments a similarly named preserve in Austin called the Balcones Canyonlands Preserve.
The Edwards Group is a geologic group in Texas. It preserves fossils dating back to the Cretaceous period.