The Geology P.A.G.E.: Geology of the National Parks in Pictures - San Antonio Missions National Historical Park

My next post about the Geology of the National Parks Through Pictures is from when I lived in Texas and we drove around exploring the national parks of the state.

You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website Dinojim.com.

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San Antonio Missions National Historical Park

San Antonio Missions is comprised of four missions that stretch out to the south of San Antonio along the San Antonio River. Starting north to south the missions are Mission Concepción, Mission San José, Mission San Juan, and Mission Espada. A fifth mission in the area, Mission San Antonio de Valero, better known as the Alamo is not part of the park but likely has a similar building history.

Mission Concepción

Built in the late 1700's, all of the missions seemed to have been built from the same building stones that were all available within the region. It appears that there were three primary stones chosen in the construction. Near the Mission Concepción is a quarry that was used to obtain the walls for the missions. The stones found here are a calcareous sandy tuffaceous limestone.

Mission Concepción

McDowell, 1997 recounts the historic observations of these building stones from Ferdinand Roemer when he traveled to the San José Mission in 1846. He stated that one of the stones is ...

"... a light, porous, tufaceous limestone or travertine, which is also found in many parts of Germany, ... where it is valued highly as a building material on account of its lightness. This stone formation finds its particular origin in the deposits of springs containing lime. The cupolas and arched ceiling of the churches in the Missions are built of this material."

Mission San José

The carved elements of the building however come from a limestone that is much softer and purer. Romer stated this about that stone:

"The other stone used is a greenish gray limestone, containing clay, which has the peculiar property of being almost soft enough to be cut with a knife when taken from the quarry, but later hardens when exposed to the air. This peculiar mineralogical product is mentioned in several writings as being found in the region of San Antonio. This limestone, whose geological age can be determined by the numerous fossils, - particularly species of the family Exogyra,-enclosed in it, belongs to the Cretaceous formation and is found in several places in the neighborhood of San Antonio."

Mission San José

Based on the inclusion of Exogyra (a Cretaceous age oyster) as well as other factors and fossils within the limestone, it can be assumed that this ornamental limestone was from the Austin Chalk Group, an Upper Cretaceous formation deposited between 89 and 84 million years ago. Formed in the shallow to deep marine deposits along the northern edges of the Gulf of Mexico, the Austin Chalk Group is a series of different formations of which the chalk itself is only part. In the chalk, the majority of the rock is almost solid fossils with Exogyra only being a small percentage. The majority of the fossils are those of coccoliths, which are the microscopic shells of organisms called coccolithophores.

Mission San Juan

The limestone that had been used for the majority of the building's construction was also used as a mortar to cement the building blocks together since when wetted, the limestone dust dried hard as concrete.

Mission San Juan

There is a possibility that other local stones were also used including the local Anacacho Limestone, Pecan Gap Chalk, and Edwards Limestone.

Mission Espada

The final distinct building stone is a red sandstone. Although I can't find a positive identification for the sandstone, the sandstone used in the building is strongly cross bedded and may perhaps be the local Escondido Sandstone or Indio formation.