Emilie & Albert
Friedrich Wilderness Park
    21395 Milsa Road | San Antonio TX | 78256

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The Framework of Friedrich Park

The balcony effect caused by alternating layers of hard and soft limestone. It inspired the Spanish word "Balcones" as the name for the escarpment and fault zone that extend through the region.

Structural geology deals with how a rock is deformed after it has been deposited. With enough time and pressure, rocks can be tilted, bent, folded like ribbon and turned end over end. When the pressure comes too fast, the rock breaks; how fast is "too fast" depends on the characteristics of the rock. The breaks can form as hairline joints where no movement has occurred. Faults are breaks where rocks on one side of the break have moved up, down, sideways, diagonally, or apart relative to the other side; thrust faults are where sheets of rock, even as thick as mountain ranges, are shoved up and over other rocks.

The structure in Bexar County is dominated by the Balcones Fault Zone. It is a series of fractures in the earth's crust whose blocks drop to the southeast. From Bexar County, the fault zone extends approximately 160 km to both the west and northeast toward the respective cities of Del Rio and Waco. The Balcones Escarpment is a result of this faulting and is the most dominant and well recognized structural feature in central Texas.

Balcones faulting probably began near the end of the Cretaceous Period about 65 million years ago, but most of the movement occurred in the early Miocene Epoch about 20 million years ago. The fault zone developed along a hinge line between sediments deposited on stable rock (the eroded remnants of an ancient mountain range) and the sediments subsiding into the Gulf of Mexico Basin. The average width of the fault zone is 20-25 km. Total drop across the fault zone in Bexar County is 760 m, with individual fault drops measured as much as 180 m. Most major fault displacements in the area are about 10-15 m, and many faults with less than 3 m of throw do not appear on geologic maps due to difficulty in mapping them. From Friedrich Park, southeast along IH-10 to just south of FM 1604, a series of faults add up to about 275 m of drop.

While faults are the fractures most easily mapped, joints are by far the most abundant structures in the area and are generally formed by stresses associated with faulting. In Bexar County, most fractures trend northeast to southwest and turn more north-to-south in the northeast part of the county as the fault zone turns in that direction. No major faults are known to cross Friedrich Park. The rocks in the area are fairly flat-lying and show no significant folding.

For More Information:

Darwin Spearing's Roadside geology of Texas, clearly explains how the Balcones Fault Zone formed. This book is widely available.

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