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

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The Rocks 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.

Rocks are described by their physical characteristics or lithology. The three basic types of rock are: igneous- formed by the cooling of molten material; sedimentary- formed by the deposition, compression, and natural cementation of materials; and metamorphic- igneous or sedimentary rocks that are physically or chemically altered by high temperature and/or pressure.

In Bexar County, the rocks are sedimentary in origin and generally become younger in age as you move from north to south. Geologists like to describe rocks in oldest to youngest order, since the character of younger rocks may partly depend on the older, underlying rocks.

The rocks are classified by age and then in sets of similar type. The broadest set is known as a group. It is made up of formations, which in turn are comprised of members. Formations are the most commonly used subdivision. These units are named for features or for locations where they were described and their characteristics are most typical. Variations occur and not every feature is present everywhere within a unit. If a rock varies enough from where it was described, it is named and described as a different unit.

As the rocks are studied in more detail, new names are assigned. If you read some old geologic reports for the Bexar County area, you may find names that are no longer used. For example:

• The Pearsall Formation is now divided into the Cow Creek Limestone and the Hensel Sand, and the Hensel is mapped as the Bexar Shale in Bexar County.
• The Walnut Formation and Comanche Peak Limestone are now included within the Edwards Limestone (at least in the Bexar County area).
• In recent reports you'll find that the Edwards Limestone is described by its two component formations, the Kainer and Person.
• The Grayson Shale is now called the Del Rio Clay.

Following is the list of rock units exposed in and near Bexar County. Detailed information is provided on those units of particular importance to Friedrich Wilderness Park.

Sedimentary rock groups, formations, and members are often identified by the presence of certain fossils that only or mostly occur in those units. Fossils are evidence of past life. They may not contain any remains of the animals that lived long ago. In most cases, all parts of the animals have been replaced by minerals or removed to leave impressions in the rock that show the shape of the animal or part of the animal. . Knowing that certain animals only live under certain condition helps tell geologists the conditions that existed when a rock was deposited.

Two rock formations occur in Friedrich Wilderness Park: the Glen Rose Formation of the Trinity Group and the Kainer Formation of the Edwards Limestone Group. The upper member of the Glen Rose outcrops throughout most of the park and the Kainer is found at higher elevations, capping the hills.

The Glen Rose and Kainer formations were deposited in a broad shallow sea, 100-114 million years ago during the Cretaceous Period. Deposition extended southward from an ancient shoreline in the Llano area, located 50 km to the north. The Glen Rose Formation is part of an assemblage of sedimentary rocks known as the Trinity Group. Trinity-age sediments reflect three periods of the sea expanding inland; the Glen Rose was deposited during the last period.

Coral reefs were deposited to form the lower unit of the Glen Rose's lower member; these grew during a time of high water circulation and energy, factors needed to sustain them. Water depth was determined be no less than 5 m. In contrast, the thicker overlying units of the lower member were deposited in a very shallow, tidal flat that was dry land briefly and intermittently. These strata have given the Glen Rose Formation notoriety among the general public due to its abundant dinosaur tracks, although none are known at Friedrich Wilderness Park. A 3-6 meter thick zone with many Salenia texana echinoid (sea urchin) fossils occurs near the top of the lower Glen Rose and indicates a return to freely circulating conditions. However, this soon changed to a restricted, saline basin at the overlying Corbula bed. This thin bed of rice-like fossils marks the top of the lower member of the Glen Rose. The lower member is not exposed in Friedrich Park, but in the Leon Creek and Cibolo Creek valleys to the north.

Upon crossing from the Glen Rose's lower to upper member is a gypsum bed, during whose deposition not even the saline-tolerant Corbula could survive. Water circulation episodically improved and worsened, creating one other significant gypsum bed in the upper Glen Rose. Studies of the rock and its fossils indicate the restricted circulation was a product of a warm, arid environment. The upper Glen Rose is the lowermost rock unit exposed in Friedrich Wilderness Park.

Shallow to tidal marine sedimentation persisted throughout the deposition of the Edwards Limestone. The onset of Edwards time was marked by subsidence and slight deepening of the sea which contained the Trinity deposits, creating a shallow, open marine environment. Tidal flats developed and later formed gypsum-filled basins. Sea level rose to inundate the gypsum deposits with layers of lime mud. As the sea water deepened, limestone deposition decreased but increased in clay content. Shallow conditions followed with a renewed period of gypsum deposition, which was followed for the remaining interval of Edwards time by deposition of lime mudstone. The Edwards presently can be found in Friedrich Wilderness Park on hills atop the Glen Rose Formation. A broad stretch of Edwards occurs a short distance south of the park along the Balcones Fault Zone.

The Edwards Limestone Group is a hard, crystalline, fossiliferous, and cavernous rock that is locally about 137 meters thick. It is subdivided into the Kainer Formation at the base, with ascending Basal Nodular, Dolomitic, Kirschberg, and Grainstone members, and the Person Formation at the top, with ascending Regional Dense, Collapsed, Leached, Marine, and Cyclic members. The lower 20 m of the Dolomitic Member of the Kainer Formation cap the highest hills at Friedrich Wilderness Park. The Basal Nodular Member, at the base of the Kainer, is equivalent to the Walnut Formation described by earlier geologic studies. It is 15 meters thick and underlies the middle to higher elevation slopes at the park. Fossils of Turritella snails are often found in the Basal Nodular Member and rudistid corals are a common fossil in some of the others members of the Kainer.

The Glen Rose Formation underlies the Kainer and is about 125-137 meters thick. Recent detailed mapping near Friedrich Wilderness Park at Camp Bullis has divided the upper member of the Glen Rose into five intervals. Intervals A and B are exposed at Friedrich Wilderness Park. Interval A is 37 meters thick and is composed of alternating and interfingering, medium-thick beds of mostly limestone mudstone, packstone, and wackestone. It underlies most of the park's lower slopes, beginning just above the parking lot area and extending about halfway up the hillsides. Interval B has a total thickness of 46 meters, but only the upper 4 m are exposed at the park. They extend from just above the parking lot down to the east and down slope of the park, and are composed of mudstones, packstones, and marls. Few fossils occur in the A and B intervals; when found, the fossils are usually the clam Procardia texana and the gastropod Tylostoma.

Overlying the rocks are thin soils. Tarrant series soils occur at the highest and lowest elevations of Friedrich Wilderness Park, and between them is a mix of Brackett and Tarrant soils. These soils are dark brown to black, poorly permeable, stony, usually less than 1 meter deep, easily eroded, and occasionally absent. They are thicker and more continuous along the valley walls. While soils are often overlooked when people examine the area's geology, they are a product of the weathered bedrock and so their distribution is directly related to the bedrock. They are important to the distribution of plants and animals.

For more information:

General geologic mapping and descriptions of geologic units in the Bexar County area are provided by Vigil Barnes' 1983 Geologic atlas of Texas: San Antonio sheet, and Edward Collins' 2000 Geologic map of the New Braunfels, Texas 30 x 60 minute quadrangle: geologic framework of an urban-growth corridor along the Edwards Aquifer, south-central Texas (Miscellaneous Map No. 39) both published by the Bureau of Economic Geology at The University of Texas. http://www.beg.utexas.edu/
Detailed mapping of the Edwards Limestone in Bexar County is provided in Geologic framework and hydrogeologic characteristics of the Edwards Aquifer recharge zone, Bexar County, Texas, written by William Stein and George Ozuna in 1995 as U.S. Geological Survey Water-Resources Investigation 95-4030.
Detailed mapping of the Glen Rose Formation has not been published for Bexar County but is available for nearby Camp Bullis through Allan Clark's 2003 Geologic framework and hydrogeologic features of the Glen Rose Limestone, Camp Bullis Training Site, Bexar County, Texas, published by the U.S. Geological Survey as Water-Resources Investigations Report 03-4081.

Stratigraphic Column for Cretaceous Rocks in the Bexar County Area

* given thicknesses are averages for the area, some variation occurs in the Bexar County area.
-------------- = conformable contact (upper unit deposited immediately after the underlying unit)
~~~~~~ = unconformable contact (upper unit deposited after underlying unit was partly or completely eroded)

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