St. Peter Sandstone:
St. Peter sandstone is a sedimentary rock found in several midwestern states, including Missouri. It consists of poorly cemented quartz sand grains, although there are trace amounts of feldspar. St. Peter sandstone was created in a marine depositional environment many years ago due to tidal fluctuations and storms when much of North America was covered in a shallow sea. The main building stone for the Basilica of St. Louis, King of France’s pillars, is this type of sandstone. In some specimens, nodules can become embedded in the sandstone. A nodule is a circular mineral embedded in the parent rock. St. Peter sandstone can contain calcite nodules.
What is Honeycomb Weathering?
Weathering is the breaking down of rocks, soil, and minerals into sediment. Mechanical weathering occurs in the breaking down of rocks via water, ice, snow, wind, etc. Honeycomb weathering is a special type of mechanical weathering that produces patterned cavities, known as tafoni (see photo below). It is most common in salt-containing environments, such as coastal zones and rivers. As rivers, such as the Mississippi and Missouri, flow toward the Gulf of Mexico, they transport trace amounts of dissolved salts. Rain is the most common agent of erosion that causes the rock to break down and wash eroded salt particles into waterways chemically. Additional factors contribute to honeycomb weathering, including wind corrosion and biogeochemical weathering by lichens. Most commonly, researchers have advocated salt weathering as the primary explanation for the formation of honeycomb weathering. Currently, it is considered polygenetic in origin, resulting from the complex interaction of physical and chemical weathering processes.
Logging Tasks:
- Describe the appearance and color of the St. Peter sandstone.
- Scour the basilica's pillars and/or sides for a calcite nodule. Do the calcite crystals appear rectangular, pyramidal, or spherical?
- Search the building stone for any signs of honeycomb weathering. Describe what you see. Do the tafoni holes appear smooth or rough in texture? What is the average diameter of the holes?
- The sandstone used for the basilica was mined locally near the Mississippi River. Based on your observations about this part of the state/St. Louis, why do you think that sedimentary rocks are prone to honeycomb weathering?
- As of June 2019, earthcaches now have required photo-logging tasks. Please provide a photo of yourself, your GPSr, or a personal item that proves that you have visited this site. Please post this in your log.
Sources:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/111/8/1250/183535/Origins-of-honeycomb-weathering-The-role-of-salts?redirectedFrom=fulltext
https://www.journals.uchicago.edu/doi/10.1086/624173
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