   |
|
 |
 |
     |
|
 |
|
|
The Edwards Aquifer is located in the south-central part of Texas. The aquifer extends from near Temple, in Bell County, south to San Antonio in Bexar County, then arcs west toward Del Rio in Kinney County. Around 100 million years ago, during the Cretaceous Period, central Texas was covered by a warm, shallow sea. This sea supported numerous microorganisms, coral and species of now extinct shellfish. As these creatures died, their skeletons, and carbonate minerals precipitated from the seawater, were deposited as sediments on the floor of this sea. These sediments would accumulate and become compacted to form the various limestones of central Texas.  Over geologic time, the land surface would rise and subside, with different sediments and different creatures accumulating each time the land surface was flooded by rising sea levels. Each time the surface would rise, water would drain from the surface to carve rivers and streams through the layers of limestone. Water is also able to dissolve many limestones very readily, especially when combined with carbon dioxide from the atmosphere and the organic acids of plant material on the earth's surface that decay as they die. As water dissolves the limestone material and transports it away, it leaves behind the holes and voids within the limestone layers that become caves. The Edwards Aquifer is named for a group of limestone formations having similar characteristics. Below the Edwards lies a relatively impermeable layer that retards the further movement of water downwards. Above the Edwards, a relatively impermeable layer was also deposited. Eventually, powerful earthquakes and volcanism shattered these limestone layers. Faulting and fracturing broke and shifted these layers, lifting large blocks of land while lowering others. The sea has subsided and the faulting has become dormant. Water has eroded the land from the inside and out creating a lattice-work of different limestones exposed at the surface, while allowing water to be trapped in between rock layers, creating the Edwards Aquifer. The aquifer can be divided into three subregions; the San Antonio (or southern) segment, the Barton springs segment, and the northern Edwards. The Northern Edwards extends from the Colorado River in Austin to the Northern extreme of the Edwards Limestone in Bell County. The San Antonio segment extends from a groundwater divide near the City of Kyle, about 20 miles south of Austin, to near Del Rio in McKinney County. Major outflows in this region include Comal, Hueco, and San Marcos Springs; three of the most significant springs in Texas. The last segment is located between the groundwater divide in Kyle to the Colorado River. The major outflow of this segment (the Barton Springs segment) is, of course, Barton Springs. Barton Springs is near and dear to many people in the Austin area for its' cool, waters and Barton Springs Pool. |
