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

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The Shape 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.

Geomorphology is the study of the shape of the land. Why are some areas mountainous, swampy, flat with deep canyons, or flat with tall pinnacles? Geomorphologists examine the forces of erosion as they interact with the lithology and structure of the rock to sculpt landscapes.

The Balcones Fault Zone is the boundary between the relatively flat Edwards Plateau to the north and the relatively flat Gulf Coastal Plain to the south. The Texas Hill County region is where streams have cut a rolling terrain along the edge of the Edwards Plateau as they flow down onto the Coastal Plain. The steep canyons to the north formed in part because of the rapid downcutting but also because rocks are harder to the north and can support cliffs and steep terrain. The rocks of the Coastal Plain are weaker, more easily eroded, and form few and short cliffs or steep areas. This low relief area also allows streams to meander more widely and flood far past their banks more readily.

The displacement caused by faults allows rocks that are normally at higher elevations to be located at lower elevations. In Bexar County, a seemingly reverse stratigraphy occurs. Normally, younger strata occur at higher elevations on top of older rocks that are exposed at lower elevations. However, Balcones faulting has dropped the rocks to the south, so that after erosion, the lower units are exposed at higher elevations to the north where little faulting occurred, and the younger units occur where substantial faulting has dropped them to lower elevations.

The terrain of Friedrich Park is comprised of gently sloping ridgetops that slope steeply down to nearly level valley floors east and southwest of the property. The elevation of the park ranges from a maximum 436 m above mean sea level at a hilltop in the west-central part of the park, to 348 m at the park's northeast corner. Almost all of its topography is presented on the Van Raub 7.5' U.S. Geological Survey topographic quadrangle, with a narrow sliver appearing on the Camp Bullis 7.5' quadrangle.

The contrast between the nearly flat valleys and ridgetops with the steep hillsides and valleys is a reflection of the underlying rocks. The rocks of the upper member of the Glen Rose Formation are relatively easily eroded compared to the rocks of the Kainer Formation. Changes between hard limestone and soft clay beds in the upper Glen Rose have resulted a distinctive stair-step topography, where roughly flat areas occur on soft rocks and rise vertically at beds of hard limestone. Thicker soils form over the softer rocks, allowing more plant growth than can be seen as horizontal bands of lusher vegetation when viewed from a distance.

The Kainer's Dolomitic Member is highly resistant to erosion and forms a hard cap over the softer underlying units. As a result, the softer units are protected from erosion by the Dolomitic and are easily eroded to form steep hillsides in adjacent areas where the Dolomitic is not present.

Most of central Texas and all of Friedrich Park occurs in a karst terrain. Most landscapes form by mechanical erosion, by streams cutting though rocks and sediments to form valleys and intervening hills. Karst landscapes form primarily by water dissolving the limestone. Water entering the ground is naturally charged with carbon dioxide from the atmosphere and soil to form carbonic acid. Over millennia, this weak acid dissolves down the limestone surface but especially enlarges fractures and bedding planes as it moves underground. As the openings become larger, water drains more efficiently.

These increasing volumes of water then enlarge the openings at faster rates, a process that self-accelerates. Eventually, one flow path forms a conduit that extends from where water enters the ground to where it discharges from a spring. The conduit then dominates the local drainage pattern, captures flows from smaller channels, and when it becomes large enough for human exploration, the conduit is called a cave.

Typical karst features include caves, sinkholes, underground streams, and solutionally enlarged fractures and bedding planes. While karst features in some parts of the world can be very large, up to tens of kilometers in length and hundreds of meters in depth, karst features in Texas are relatively small. No caves are known at Friedrich Park. In Texas, the minimum size for a cave requires it be humanly enterable for 5 m in length and/or depth. A solutionally enlarged bedding plane, from which a spring flows intermittently, is the largest known karst feature at the park; it is only about 3 m long. Karst features more commonly found at the park are solutionally enlarged fractures and a variety of karren, groves, channels, tiny pits, basins, and related shapes formed by water dissolving the bedrock surface.

For more information: The American Geological Institute offers Living with Karst, edited by George Veni and Harvey DuChene in 2001, a detailed overview on karst, how it forms, the resources it offers, the problems unique to it, and solutions to those problems. This publication was written for educators, land managers, and non-specialists who want a good understanding of karst landscapes.

The Texas Speleological Survey is a non-profit organization that compiles and publishes data on Texas caves and karst for research, education, and conservation purposes. The Caves and Karst of Texas, edited by William R. Elliott and George Veni in 1994, is a comprehensive overview of Texas karst areas and major Texas caves. Several other publications on Texas caves and karst can also be ordered.

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