Sink or Swim EarthCache

Austin Nature Preserves Rules

"A City of Austin Nature Preserve is a place containing significant natural communities, species, or unique natural features, and which is managed in perpetuity for the primary benefit of those features, and to provide educational, scientific, and aesthetic benefits." - Nature Preserves Task Force, 1984

There are several things visitors can do to help sustain these fragile habitats. For a list of park rules, click here. To ensure this earthcache remains on this site and as a learning tool, please conform to all of the Austin Parks & Recreation system rules.

The Edwards Aquifer

The Edwards Aquifer is a karst aquifer. It covers an area of approximately 4000 square miles stretching from the Rio Grande River, near Del Rio, to the town of Salado in Bell County. On the surface the Edwards Aquifer is comprised of three areas: the catchment, recharge, and artesian zones. The catchment zone collects rainfall and directs it to the recharge zone. Surface runoff enters the Edwards Limestone through fractures, sinkholes, and sinking streams within the recharge zone. This area is considered an unconfined aquifer because water can enter and exit freely. The artesian zone is a confined section of the aquifer. The Del Rio Clay acts as a barrier, holding water in and preventing further recharge. In some places in this zone, groundwater is under pressure and is released in artesian springs.

The Edwards aquifer is the most transmissive of all the aquifers in Texas and Oklahoma. Large discharges from springs and from flowing and pumped wells attest to the highly permeable nature of the aquifer. The area underlain by the Edwards aquifer is a combination of agricultural and ranch land and areas of dense population, including the cities of Austin in Travis County and San Antonio in Bexar County. It has been designated a sole source aquifer by the U.S. Environmental Protection Agency and is the primary source of water for San Antonio, America's eighth largest city. The aquifer is separated into three parts by a groundwater divide and the Colorado River.

Following deposition of Cretaceous rocks, tectonic movement caused the relative uplift of the Edwards Plateau and subsidence of the Gulf of Mexico, and a number of en echelon, northeastward-trending faults formed along the Balcones Fault Zone. The Edwards aquifer is generally coincident with the fault zone. The downdip boundary of the aquifer is largely fault controlled. As a result of the faulting, the chemical quality of the water in the Edwards aquifer can change abruptly in a very short distance across a zone often referred to as the "saline-water line."

As streams cross the Balcones Fault Zone, water percolates downward along the faults where permeability might be greatly enhanced by partial dissolution of limestone. Secondary sources of recharge are direct infiltration of precipitation that falls on aquifer outcrop areas, internal flow of groundwater from the Trinity aquifer where the Edwards and the Trinity aquifers are juxtaposed, and upward leakage from the underlying Trinity aquifer where an upward vertical head gradient exists. Direct recharge to the aquifer can be quite rapid through sinkholes.

The Goat Cave Nature Preserve

The eight acre Karst Preserve sits atop a portion of the Edwards Limestone containing numerous water-formed caves, sinkholes and smaller openings. "Karst" is the geological term for this type of honey-combed limestone. The Edwards Aquifer is water contained within the karstic limestone of the Edwards formation. The Edwards Aquifer Recharge Zone is a two mile wide area in South Austin where the karstic portions of the Edwards Limestone break the surface of the ground. Within this relatively small preserve one can experience numerous caves, sinkholes and porous limestone known as karst.

Karst is a special type of landscape that is formed by the dissolution of soluble rocks, including limestone and dolomite. Karst regions contain aquifers that are capable of providing large supplies of water. Common geological characteristics of karst regions that influence human land uses and water resources include caves, sinkholes or sinks, sinking streams, and springs.

A geographic sink is a depression in land where water collects with no visible outlet. Instead of discharging, the collected water is lost due to evaporation and/or penetration (water sinking underground to become groundwater in an aquifer). Sinkholes can range in size from a few feet or meters to over 100 meters (300 feet) deep. They've been known to "swallow" cars, homes, businesses, and other structures. A sinkhole can even collapse through the roof of an underground cavern and form what's known as a collapse sinkhole, which can become a portal into a deep underground cavern.

The first sink you come to in this area is Wade Sink. Wade Sink may be explored, but you have to remember that you are sharing this habitat with scorpions, spiders and snakes, and that they have the right of way. The entrance to the sink is fairly small and you will have to shimmy your way through.

Farther up the trail is Goat Cave, the main karst in the area from which the preserve draws its name. This cave serves as the home of a colony of bats and the city has started to take steps to restore the karst to a healthy environment for them. Years ago, in an effort to keep people from falling into the 28 foot drop at the entrance a large metal grate was placed over the opening. Today access to Goat Cave is restricted due to its fragile nature and may only be accessed by permit with staff and or approved cave ethics training. To learn more, contact their office at (512) 974-8875

NOTE: To claim your find, please answer these questions:

1. If you can't read the park entrance sign you may have to do a little research. The below ground sinks and tunnels are connected into a vast sub-terranean formation stretching from Austin to what city?
2. You'll have to do a little measuring here and the numbers do not have to be exact; the measurements are approximate in feet. The entrance hole at ground level of Wade Sink is approximately ___ feet by ___ feet in diameter.
3. After Wade Sink is another sink. Name it.
4. While you can crawl through and explore Wade Sink, what is noticeably different about this second sinkhole? Please describe what you see physically from the trail; access inside the cave is prohibited.
5. This last question will require you to hike the entire trail at the preserve, but be sure and stay on the trail. Doing this task takes about 30 minutes, but will give you the opportunity to see the whole preserve. What is the distance between the cache coordinates provided and the "turnaround" or end of the trail at Goat Cave Nature Preserve?

Send your answers to me via email. LOGS WITHOUT VERIFYING EMAIL WILL BE REGRETFULLY DELETED. Also, do not post the answers here, even encrypted. Photos are optional, but feel free to add them to your log to let others know what a beautiful place this is.

The coordinates will take you to the beginning of the public trail. The trail and first cave (sink) is open to the public, but be respectful of the area and stay on the trail. Do not go into the restricted areas. In other words, FOLLOW THE RULES!!