Recent Alluvium

Gravel, sand, silt, and clay. Most gravel clasts have limestone composition. Occurs in meanders of Blanco River, Sink Creek, and Purgatory Creek. Also a broad area at the Balcones Escarpment under the city of San Marcos. Generally less than 20-30 ft thick along streams.

Lower Terrace Alluvium

Gravel, sand, silt, and clay. Most gravel clasts have limestone composition. Generally 30 feet thick. Occurs primarily as broad terrace near the Blanco and San Marcos Rivers, which is now cutting through the former floodplain alluvium into bedrock. Often has resistant caliche cap where exposed in the riverbank.

Upper Terrace Gravel

Gravel, sand, silt, and clay. Occurs in small patches at higher elevations at and around the Balcones Escarpment, with one occurrence several miles southeast of the scarp. May be the “Uvalde gravel” recognized elsewhere along the escarpment.

Taylor Group

Described as “blue-gray nodular, locally chalky marl” with a thickness of about 300 ft for Hays County (DeCook, 1963 ). Consists generally of smectitic (montmorillonitic) mudstone. Subdivided into three formations (Sprinkle, Pecan Gap, and Bergstrom in ascending order) based on relative calcium carbonate content. Pecan Gap and Bergstrom crop out in map area. Locally contains large specimens of Exogyra ponderosa. Forms unstable slopes in natural steep-slope conditions (e.g., near the San Marcos River) and where excavated for development. Supports growth of mesquite and juniper trees. Occurs over a large area southwest of the Balcones Escarpment in the southern half of the map area.  (Not include; incorporated below)

Bergstrom Formation

Montmorillonitic mudstone. Relatively less calcium carbonate than underlying Pecan Gap Formation. See lithologic description for Taylor Group.

Pecan Gap Formation

Montmorillonitic mudstone. Relatively more calcium carbonate than overlying Bergstrom Formation. See lithologic description for Taylor Group.

Sprinkle Formation

[Remove this map unit. Does not crop out in the map area (faulted out)]

Austin Group

Characterized as “light-gray marly or chalky limestone and tan marly fissile shale” with a thickness of 160-200 ft for Hays County (DeCook, 1963). Consists of argillaceous or chalky limestone in map area. Contains small seams and crystals of pyrite or marcasite in some areas. Upper contact not exposed in area. Subdivided into several formations, but their lithologies are not sufficiently distinct for mapping individually. Occurs in broad area in the northeast quarter of the map area

Eagle Ford Formation

Described as “blue calcareous shale and gray arenaceous, bentonitic limestone for Hays County (DeCook, 1963). Consists of three layers having a total thickness of 20-25 ft in San Marcos area – lower bentonitic (montmorillonitic) shale (6-7 ft), middle calcareous flaggy sandstone or siltstone (4-5 ft), and upper shale (12-13 ft). Lower and upper contacts are distinct and easily mappable on air photos. Subject to slumping were it is excavated for development. Occurs generally with Georgetown, Del Rio, and Buda Formations in a band extending from the northeast to the southwest corner of the map area.

Buda Formation

Characterized as “tan and gray massive hard nodular limestone” with a thickness of 30-60 ft for Hays County (DeCook, 1963). Observed in the field to consist of hard, nodular limestone in lower part and hard, thick-bedded limestone in upper part, with a thickness of 45 ft. Readily mappable lower and upper contacts on air photos. Supports oak-juniper assemblage. Occurs generally with Georgetown, Del Rio, and Eagle Ford Formations in a band extending from the northeast to the southwest corner of the map area.

Del Rio Clay

Described as “Blue-gray to tan gypsiferous, ferruginous marly shale” with a thickness of 40-60 ft for Hays County (DeCook, 1963). Characterized by gypsum veinlets, pyrite or marcasite, and abundant specimens of Ilmatogyra arietina. Clays are dominantly illite and kaolinite in unweathered state, but the illite is converted in the weathered zone into plastic montmorillonite. About 45 ft thick in San Marcos area. Subject to slope failures during construction. Distinguished in the field by predominance of mesquite trees in the clayey soil. Lower and upper contacts readily mappable on air photos. Occurs generally with Georgetown, Buda, and Eagle Ford and Formations in a band extending from the northeast to the southwest corner of the map area.

Georgetown Formation

Characterized as “light-gray and white argillaceous nodular limestone and tan calcareous shale” for Hays County (DeCook, 1963). Thickness reported as 25 to 35 feet near San Marcos (DeCook, 1956 ). Sharp and readily mappable contact (on air photos) with underlying Person Formation. Supports oak-juniper assemblage. Occurs generally with Del Rio, Buda, and Eagle Ford Formations in a band extending from the northeast to the southwest corner of the map area.

Edwards Group, Undifferentiated

Reported as “gray dolomitic siliceous massive honeycombed limestone” in upper part and “light-gray argillaceous, nodular limestone” in lower part for Hays County (DeCook, 1963). Thickness reported at 430-450 ft for San Marcos area (Rose, 1972 ). Divided by Rose (1972) into lower Kainer and upper Person Formations, separated by the “Regional Dense Member”. Comprises principal aquifer in the map area and throughout central and southwest

Texas. Occurs under typical hill country landscape in broad band in west and northwest part of the map area, with predominantly oak-juniper assemblage. Common sinkholes and other karstic solution features.

Person Formation

Lithology and other characteristics as described for Edwards Group – similar to underlying Person Formation. Distinguishable as mapping unit only when lowest bed (“Regional Dense Member”) is present. Thickness reported at 130 to 150 ft in San Marcos area (Rose, 1972).

Kainer Formation

Lithology and other characteristics as described for Edwards Group – similar to overlying Person Formation. Distinguishable as mapping unit only when lowest bed of Person Formation (“Regional Dense Member”) is present. Thickness reported at 300 ft in San Marcos area (Rose, 1972).

Walnut Formation

Lithology of “blue-gray sandy or calcareous clay; light-gray to white argillaceous nodular limestone” and thickness of 5 to 15 ft reported for Hays County (DeCook, 1963). Thickness reported also at 40 ft (Davis, 1962 ). Abundant occurrence of Exogyra texana in Hays County. Crops out only in Blanco River canyon near northwest corner of map area. Mapped with overlying Kainer Formation on this map.

Glen Rose Formation

Characterized as “hard limestone alternating with argillaceous marl” with a thickness of 500-900 ft for Hays County (DeCook, 1963). Observed as finely crystalline dolomite interbedded with dolomitic limestone and marl in map area. Upper 35 to 45 feet exposed in map area. Crops out only in Blanco River canyon near northwest corner of map area.