Hills of Friedrich Park

Friedrich Wilderness Park lies on the Balcones Escarpment at the edge of the Balcones fault zone that separates the Edwards Plateau from the South Texas Plains. Although the fault is now quiet, it lies in an area of ancient and repeated geological activity.

The Balcones fault zone parallels a portion of the Ouachita line. This important geological boundary forms a sweeping 'S' curve through the center of the state, running from east of Dallas, down through Austin and San Antonio, and westward through the Big Bend of the Rio Grande. This line follows a deep zone of weakness in the earth's crust, and thus has been the location for several episodes of continental collision and separation beginning about a billion years ago.

This line was the site 300 million years ago of the ancient Ouachita Mountains formed as the earth's continents collided to form the supercontinent Pangaea. The Ouachita Mountains were part of a long range that includes the current Appalachians, Ouachita Mountains (still present in southern Oklahoma), and the Caballos Mountains around Marathon in west Texas.

Over the following 200 million years, these mighty mountains were eroded nearly flat into the Cretaceous seas. Pangaea was pulling apart to begin forming the continents we know today. About 60 to 100 million years ago as the continents drifted apart and the Gulf of Mexico began to form, it was this Ouachita line that marked the edge of the North American continent and the shores of the Gulf. About 10 million years ago the earth heaved up again to raise the older Cretaceous strata to nearly 2,000 feet above sea level, forming the Edwards Plateau. The fracture along which this last uplift occurred is the Balcones fault zone.

The Edwards Plateau has been rapidly worked by aggressive erosion. Runoff water, streams and rivers have continually cut upward and downward. Since its uplift, about half of the plateau has been removed in this manner. The Northwest half remains a high, flat plateau, while the southeast half is highly eroded to form valleys and low, rounded hills, and is known as the Texas Hill Country.

At Friedrich Wilderness Park most of the rocks and rock strata you will see are of the Glen Rose formation. These limestones have been laid down in alternating harder and softer layers, which erode at different rates. This produces a stair-step contour to the local hills. The younger Edwards limestone formation has for the most part eroded away, yet small remnants may be found at the very tops of the hills.

Limestones are formed from the deposited remains of marine organisms which had calcium based shells or skeletons. Small amounts of another rock called chert are also found in the region. Chert is also a sedimentary rock formed from the remains of marine organisms, but those whose hard parts were silicon based. Chert is a hard, glassy rock, often bluish-brown in color. It is the rock used by aboriginal Americans to make their stone tools.

 
Photo by Rob Badgett ©   Photo by Rob Badgett ©   Photo by Rob Badgett ©
 
  The Vista Loop trail on the shady north side of the park follows a natural ledge in the limestone and passes through a thick forest of diverse types of trees. In addition to the predominent live oak and Ashe juniper, other trees, other trees including spanish oak, black cherry, Arizona walnut, buckeyes, and cedar elm. Other interesting trees found on the back slopes of the park include lacey oak, rusty blackhaw, carolina buckthorn, rough-leaf dogwood, and a lone mexican plum with its striking white blooms appearing late February or early March.
 
 

Essay by Eric Lautzenheiser and Bob Badgett
Additional text by Patty Leslie Pasztor and Bob Badgett

For more information about area geology, see:
Spearing, David. Roadside Geology of Texas. Mountain Press Publishing Company. Missoula, Montana 1979.
Cox PW and Leslie P. Texas Trees. A Friendly Guide. Corona Publishing Company. San Antonio Tx 1988 (Patty Leslie Pasztor is a former naturalist for Friedrich)