Tuesday, February 1, 2011

Earthquakes? In Texas?

I almost passed over the brochure for Wonder World Park in the Visitor Information Center in San Marcos (TX). Thinking it was an amusement park, I was just about ready to move on to the brochure for Dick's Classic Garage, when I noticed the phrase "Texas' Most Visited Cave" on the Wonder World brochure.

This was catchy enough, but the words "Earthquake-Formed Cave" had me hooked.

A short drive one morning brought us to the "huge underground cave, formed 30 million years ago when an enormous earthquake occurred during the creation of the Balcones Fault" (brochure notes).

The temperature was in the low 30's, and it was, there-fore, not surprising that we were the only ones on the tour into the cave that was discovered in 1893 by Mark Bevers who was drilling for water.

The first exploration of the cave occurred several days later, when Eliabeth Bevers, his wife, was throwing dish water out the back door and saw vapor coming out of a crevice. Mr. Bevers and a neighbor were able to roll a large boulder away, revealing a natural entrance to the cave.

When we entered the cave, we were given instructions unlike those we would we hear in any other cave: you may take flash photographs and you may touch sides of the cave.

Those two activities were allowed because Wonder Cave is a "dry cave," i.e., it was not formed by the solution of limestone by water.

It's a rare Tectonic cave, formed primarly by tectonic forces. And among these rare tectonic caves, the caves produced by faults are the most rare ones. In fact, Wonder World Park is a Texas State Historical Site and the only earthquake-formed cave in the nation.

But this is a result of the unique process which formed this cave. It developed along the Balcones Fault Zone (see below).

We entered the cave and began a descent that would conclude about 150 feet below the surface. In 1916, Arthur Rodgers installed the first electric lighting, handrails, and paths.

Wonder Cave is a rather small cavern with little formations. The passages are narrow and steep. This archway (photo below) was one of the more interesting formations and one of the few that required ducking our heads to pass under.

Texas is blessed with some of the nations most interesting "show" caves and thousands of "wild" caves throughout the state.

For two people who were surprised to find this one cave in Texas, we were amazed to learn that because of the varying geographical regions and features of Texas, you could literally tour the entire state by jumping from cave to cave.

In fact, the Edwards Plateau is one of the largest yet understudied Karst regions in the nation, and there are over 130 caves that have been discovered within this area with many more yet to be fully explored.

"Karst" refers to a type of terrain usually formed on carbonate rock (limestone and dolomite), where groundwater has solutionally-enlarged openings to form a subsurface drainage system, creating large cave areas in the process. In short, beautiful sub-tropical caves with large and interesting rooms, most with flowing water somewhere within the cavern.

The Balcones Fault, created by a prehistoric earth shift, is a tensional structural system that runs approximately from the southwest part of the state near Del Rio to the north central region near Waco along Interstate 35. Our guide noted that the Fault runs a total of about 1200 miles, but only about 350 miles is a true Fault; the other miles are composed of smaller fissures.

The Balcones Fault zone was most recently active about 15 million years ago. This activity was related to subsidence of the Texas Coastal Plain, most likely from the large amount of sediment deposited on it by Texas rivers. The Balcones Fault zone is not active today, and is in one of the lowest-risk zones for earthquakes in the United States.

Nonetheless, there are seismographic readings constantly being recorded--which was very reassuring to us as we walked through small passages nearly 200 feet below the surface.

The next two photos show fossils embedded in the limestone walls.

At the end of the tour, we took an elevator up to the top of the Tejas Observation Tower (90 feet above the surface) for views of the surface expression of the Fault, called the Balcones Escarpment, which forms the eastern boundary of the Texas Hill Country (photo right, looking west) and the western boundary of the Texas Coastal Plain (photo below, looking east) and consists of cliffs and cliff-like structures.

The change in elevation of the terrain is quite abrupt, with the ter-rain north of the fault ranging from 1,000 to 2,000 feet above sea level. South of the Balcones Escarpment, the terrain is generally 600 feet above sea level or lower, declining gently for about 150 miles to the coast of the Gulf of Mexico.

Earthquakes AND caves in Texas.

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