While exploring Pleasanton, I stumbled upon the Old Rock Schoolhouse which incorporates the local geology into its own construction. Here, according to the Texas Historical Marker, the building was constructed of locally quarried red sandstone. The Old Rock Schoolhouse was built in 1874 with funds pledged by citizens of Pleasanton.
At the above coordinates, you will see the Old Rock Schoolhouse which is prominent in red sandstone. Sandstone is a sedimentary rock composed mostly of quartz sand, but it can also contain significant amounts of feldspar, and sometimes silt and clay. Sandstone that contains more than 90% quartz is called quartzose sandstone. When the sandstone contains more than 25% feldspar, it is called arkose or arkosic sandstone. When there is a significant amount of clay or silt, geologists refer to the rock as argillaceous sandstone. The color of sandstone varies, depending on its composition. Argillaceous sandstones are often gray to blue. Because it is composed of light colored minerals, sandstone is typically light tan in color. Other elements, however, create colors in sandstone. The most common sandstones have various shades of red, caused by iron oxide (rust). In some instances, there is a purple hue caused by manganese.
It is likely that the sandstone that was used to construct the Old Rock Schoolhouse was obtained from the Cook Mountain Formation. This formation in Texas occured during the middle Eocene period as a result of sedimentation in a marine sea. The Eocene period last from about 55.8 to 33.9 million years ago.
The Cook Mountain formation crops out in Atascosa and Frio Counties in a belt whose entire width lies within the two counties. Many small sections of the Cook Mountain formation are found throughout the area, although, in general, the formation is better exposed in Atascosa County than in Frio County.
There are two types of deposits that can occur in sandstone. A marine sandstone deposit can occur when it is formed close to the water, shore or beach. These often contain shell and other marine fossils. A terrestrial sandstone deposit can occur when it is formed in large deserts or sand dunes. These contain no marine fossils and are much thicker in size.
Works Cited
https://www.geologyistheway.com/sedimentary/sandstone/
https://mineralseducationcoalition.org/minerals-database/sandstone/
https://www.geology.com/rocks/sandstone.shtml
https://www.researchgate.net/figure/1-Cook-Mountain-Formation-top-elevation-feet-above-mean-sea-level-which-was-prepared_fig50_348415396
https://www.texasalmanac.com/articles/geology-of-texas
https://pubs.usgs.gov/wsp/0676/report.pdf
TO LOG A FIND ON THIS CACHE YOU MUST ANSWER ALL THE QUESTIONS BELOW. YOU CAN CONTACT ME THROUGH THE GEOCACHING MESSAGE CENTER TO SEND YOUR ANSWERS.
1. "Schoolhouse Rock!" on the first line of your mesage AND I would prefer each cacher send me an individual message in the spirt of Earthcaching.
2. Describe the (a) range of colors, (b) textures, (c) and grain sizes of the sandstone.
3. What is the most predominant color you see at this location?
4. Do you see any fossils? If yes, describe them.
5. Do you think this sandstone was formed on land or with water?
6. Take a photo of you (or your signature item if you don't want to show your face) with the schoolhouse in the background. This photo MUST be uploaded to your "found it" log. Please do not send any photos through the message center as Groundspeak compresses images. Each log MUST have a unique and different photo. No two logs may contain the same photo. Sample photo below.
