Index to all pages:

Faults and Caves


The graphic above gives you an idea of the great number and complexity of the faults of the Edwards Aquifer. There's so many faults it makes you dizzy to look at them at this scale!  There are places on this graphic where many small faults just kind of form a red blob.

Large faults act as barriers or partial barriers to groundwater flow; while smaller faults and associated joints form local and regional ground-water conduits.  The graphic does not show the direction of dip, but for most of these faults it is toward the southeast.  Note how most of the faults tend to run from southwest to northeast.


Many of the normal faults of the Balcones Fault Zone are surrounded by zones of highly fractured strata.  When Loop 1604 was widened near I-10 in northwest Bexar county in the mid-80's, a road-cut was made under the railroad tracks and the ground was found to be so highly fractured and unstable that retaining walls had to be built to keep unconsolidated material from sliding onto the road.

An abrupt change in the composition of limestone marks one of the many normal faults of the Balcones Fault Zone.  This one came to light during excavation for construction of the Target near the corner of 1604 and Bandera Road.

Large solution chamber  

A large solution chamber in Glen Rose limestone at the Cave Without A Name. This room is about 100 feet wide, 60 feet high, and 400 feet long. Almost every kid who grew up in the Texas Hill Country has stories about exploring caves, as they are very common. This particular cave has an underground river flowing through it that has been explored and mapped for over three miles.

Pit cave in NW Bexar county

This pit cave in northwest Bexar county is about 60 feet deep. There are thousands of such recharge features in the area, many of which have not been documented. Many of these features that are sinkholes today were springs 13,000 years ago when the climate was wetter and cooler. Now that the climate is much drier, the direction of flow is reversed and water only goes in. Note the weathered remnants of a large stalactite that formed when this spot was still overlain by thousands of feet of limestone.

Limestone fault 

A limestone fault on the Edwards Plateau exposed by a roadcut on U.S. 281 north of San Antonio. One can observe slickensides here. The Balcones Fault Zone contains a complicated series of such faults and fracture.

Small parallel faults in Canyon Gorge

A series of small parallel faults, each with a displacement of only inches, was revealed when Canyon Gorge was created by floodwaters in 2002 (see the Canyon Lake page for more on the Gorge).

Wonder Cave

Wonder Cave in San Marcos is the oldest commercially operated cave in Texas.  It is a dry, A-frame cave formed along one of the many faults within the Balcones Fault Zone.  Although there are not many formations typical of wet caves, it is an interesting look at the shifting and faulting that occurred during the uplifting of the Edwards Plateau.  San Marcos Springs are just a few thousand feet north of Wonder Cave. The cave was discovered in 1893 by Mark A. Bevers, who was drilling for water in what is now the parking lot, and was originally known as Bevers Cave. It was purchased in 1903 by W. S. Davis, who opened it to the public.

Fault in Wonder Cave 

A fault line is visible on the ceiling of Wonder Cave.  Geologists are not sure whether these faults formed all at once, such as in a large earthquake, or over a long period of time.

Souvenir of Wonder Cave (pdf file is 24 mb)

A 12 page brochure published by Arthur B. Rogers, who purchased the cave in 1916 for $50, a gray horse, and a saddle. He installed electric lights, paths, handrails, and ladders, and then went on to develop Aquarena Springs. It is undated, but contains an advertisement for Rogers' Spring Lake Park Hotel at San Marcos Springs, which opened in 1929, so it must have been produced sometime between then and 1958, when Rogers sold the cave to T. J. Mostyn.

Wonder Cave, 1956 postcard

A color-enhanced view of the cave on a postcard mail in July of 1956. The cave is not actually those intense shades of green and purple. The caption on the back of the card says:

Subterranean fantasies are explored by visitors traversing natural sub-surface pathways.

Well in Wonder Cave

A man-made wishing well in Wonder Cave is the lowest point in the cave, about 158 feet below the surface.  The well connects with the nearby Ezell's Cave, home to the endangered Texas Blind Salamander.  Ezell's Cave is owned by the Nature Conservancy of Texas and is closed to the public.  The pulley in the photo was used by turn-of-the-century cave explorers to get water from the lake below.  Before stairs made the walk easy, it could take 5-6 hours to spelunk down to this spot.

Robber Baron Cave

Robber Baron Cave is one of the most significant caves in Bexar county. It is a vast network of passageways underneath the area south of Loop 410 near Nacogdoches Road. It is the only known home of two endangered species, Texella cokendolpheri (a harvestman) and Cicurina baronia (a spider).

The Cave was a tourist attraction from 1926 to 1933, with a cable car ride to the entrance. During Prohibition one of the larger caverns was used as a speakeasy, and the metal door is still inside the cave. Historic graffiti covers portions of some walls.

Many young boys who grew up in the area in the 40s and 50s have tales of exploring the Cave instead of attending class. Eventually the Cave entrance was filled with trash and debris and access was difficult except for the most determined.

In 2002, the Texas Cave Management Association began a restoration project and dedicated volunteers spent five years excavating the entrance, making the cave safe and accessible, and landscaping the area. In 2008 the Cave was officially reopened, and visits can be made by appointment. For more information on the history of the cave or to schedule a visit, visit the TCMA website.

Group of Robber Baron cavers after an exploration

A group of intrepid explorers in Robber Baron Cave in 2007.

Robber Baron Cave entrance

The entrance to Robber Baron Cave, excavated by volunteers after being covered by trash and debris for decades.

Robber Baron Cave entrance

To get in the Cave, you have to be able to squeeze through a narrow 18" opening in a protective grate.

Typical Robber Baron Cave passageway

Be prepared for a lot of crawling through very narrow passageways.

Typical Robber Baron Cave passageway

The cave is sort of an underground jungle gym, with many passageways where you have to shuffle along spider-like with all four limbs on the side walls and nothing below you.

Robber Baron Cave map

A map of the cave shows how extensive the known network of passageways is. Volunteers believe there are many more that are as yet undiscovered.

Robber Baron Cave graffiti

There is a lot of graffiti in the Cave dating back to the 1800s.