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Home , Austin (building), Balcones Fault, Barton Springs, Escarpment, Mount Bonnell, Pennybacker Bridge

and the Balcones Escarpment

SOUTH WEST NORTHWEST

Edwards What is an Escarpment? Remnants BartonBarton Tom Miller Mount Bonnell Remnants Groundwater in the Zone SpringsSprings Dam Fault An escarpment is an area where the elevation of the changes suddenly forming a line of hills or Pennybacker scarps. At Mount Bonnell, you are atop the Balcones Escarpment. Bridge Springs are natural sites where groundwater

E The Balcones Escarpment runs through Central from , southwest through , Hill Country Hill Country flows to the surface. About 4 miles south of d Albert R. Davis Mount Bonnell, , the w and from there to the Rio Grande (Big River) near Del Rio. It marks the boundary between the Great DOWN Barton Springs Water Treatment a stretching west, and the to the east. The break in the landscape defines major River Terrace fourth-largest spring system in Texas, has rd GULF Plant s UP P PLAINS differences in earth resources, weather and climate, soils, vegetation, and animal habitats. East of the an average flow of approximately 22,400 la COASTAL escarpment are the prime farmlands of the Blackland Prairies; to the west are the rangelands and tea gallons per minute from the Edwards . u woodlands of the Hill Country and the Edwards Plateau. The Balcones Escarpment marks the beginning River Terrace LA R PLAIN River Terrace This spring water, issuing from faulted K em E of the American West in terms of landscape, ecology, geologic processes, and human endeavors. limestone, supplies water to Barton Springs n

T a n Pool in Austin’s . The cavernous R t A s V Physiographics Regions RIVER / aquifer provides a habitat for an endangered I S Physiographic and Subregions species of salamander that lives in these Provinces springs and nowhere else. Balcones Escarpment Hill Mount Bonnell Fault Great Plains Mount

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UU EE AA Red River AAKK P En Español L Gulf Coastal Plain L L 0 100 mi CENTRAL Fort A LOWLANDS

0 150 km Worth Spherical, aerial (drone) photo credit: El Monte Bonnell (785 pies sobre el nivel del mar y 300 pies sobre el lago Austin) domina Mount Bonnell N BP Blackland Prairies (BP) Chris Zahm, Bureau of Economic Geology I

I Present Hill N la Escarpa de Balcones y ofrece un panorama del centro de Austin hacia el este y los

A Mount Bonnell Fault Geologic History S Younger bordes erosionados del Edwards Platueau hacia el oeste. Waco L Mount Bonnell (785 ft. above sea level and 300 ft Country Quaternary of the landscape by the and its tributaries began around 15 million years ago. The Edwards Plateau once Tom Miller Dam BP P Boundaries above Lake Austin) overlooks the Balcones 3 extended unbroken across . With faulting, aggressive down-cutting by the Colorado River began. Streams sculpted the ~100,000 y

landscape, leaving isolated remnants of the Edwards Plateau and forming the Balcones Escarpment. BASIN EDWARDS Escarpment and provides a panorama of downtown Mount Bonnell Younger Older AND PLATEAU Austin L (elevation 785 ft above sea level) Quaternary Downtown RANGE Great Plains A Austin to the east and the eroded edges of the 2 Ma } Austin HC T Balcones Escarpment Neogene CENOZOIC DISPLACEMENT of rock strata across the Mount Bonnell Fault occurred about 20 to 15 million years ago. Rock layers were displaced Barton Springs S Edwards Plateau to the west. These very different San Balcones Escarpment 23 Ma } 2 upward west of this break relative to the down-dropped strata to the east. Visit the Texas GeoSign website to view the English and Spanish-language R Del Rio Antonio A VE landscapes were sculpted by the Colorado River Mount Barker Paleogene versions of this sign. http://www.beg.utexas.edu/geosign. I O R R Edwards Limestone cap i 66 Ma T E XAS o C and its tributaries to form the Balcones Escarpment COL O G O r Mount Bonnell Fault Upper Visite el sitio web de Texas GeoSign para ver las versiones en Inglés y O D Balcones a F GEOSIGN R A n along the main fault line at the foot of Mount Bonnell. d L Up-faulted base Cretaceous Escarpment e Español de este letrero. http://www.beg.utexas.edu/geosign. U ~ 100 Ma DEPOSITION of strata (layers of rock) in the Austin area occurred about 66 to more than 100 million years ago. During this period, the sea G of Edwards Limestone covered all of Central Texas. Variable marine conditions, such as depth, water temperature, and salinity produced sand, limy ooze, and clay Water treatment plant Lower 1 600 ft 600 ft

The Mount Bonnell Fault is one of many faults in the ~ Cretaceous deposits that eventually hardened into the bedrock strata that we see today. Did You Know? 145 Ma }

Balcones Fault Zone. In this area, rock formations MESOZOIC east of the are 600 ft lower than } Jurassic The Zone was active between Mount Bonnell Fault Triassic 20 and 15 million years ago. Although no major the rock formations to the west of the fault. 252 Ma Balcones Escarpment Permian N earthquakes along the Balcones Fault Zone Mount Bonnell Fault have occurred in recorded history, this does not Edwards 299 Ma Limestone Pennsylvanian mean that earthquakes could not happen again! UP GEOLOGIC TIME In fact, two small historic earthquakes Fault displacement ~ Fault displacement 323 Ma show the locations of major -- Mississippian, Physiographic maps (1873 and 1902) in Travis County may have DOWN Devonian, Mount Bonnell’s geologic history includes three major events that took place between ALEOZOIC Silurian,

been caused by small movements along the P , plains, and ranges. Ordovician, 100-plus million years ago and the present, and that shaped today’s landscape. Balcones Fault Zone.* Cambrian 0 5 mi 541 Ma 0 8 km Established in 1909, the Bureau of Economic Geology is the oldest organized research unit of The University of Texas * Source: Frohlich, C., and S. D. Davis, Texas Earthquakes, and is the Texas State Geological Survey. Scott W. Tinker serves as its director and as State Geologist of Texas. Our Source: Topographic depiction from Garner & Young, 1976, Plate VII Contour interval 20 ft (~6.1 m) Univ. Texas Press, 275 pp., 2002 Down-faulted Projected elevation at PRECAMBRIAN mission is to serve society by conducting objective, impactful, and integrated geoscience research on relevant energy, base of base of down-faulted environmental, and economic issues. Our vision is to be a trusted scientific voice to academia, industry, government, Edwards Limestone Edwards Limestone Older and the public, whom we serve. The Bureau is part of the Jackson School of Geosciences at The University of Texas EAST WEST 4600 Ma at Austin. Simplified Cross Section Age (Ma) = million years ago Source: Geologic Society of America, modified from v. 5.0 QAe7258