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 Rd. and was subsequently covered by a retaining wall.

Large solution chamber
	A large solution chamber in Glen Rose limestone - 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.
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.
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.
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.