Common rocks in Eastern Iowa:
In Eastern Iowa, most places are covered in a layer of glacial drift from the Wisconsinan Glaciation Period. This geological event occurred roughly 10,000 years ago when glaciers covered this part of the state. When glaciers receded, they left gravel, boulders, and sand from land north of Iowa (Minnesota and Canada). Below the glacial debris, sandstone and limestone bedrock are common.
Anamosa Limestone: this type of limestone is found only in Eastern Iowa. Similar to sandstone, limestone was formed due to deposition when Iowa was submerged in a carboniferous sea. Limestone is created when marine debris (such as coral and shell remains) that contain calcium carbonate lithify, forming sedimentary rock. This type of limestone can be beige to slightly yellow in color but can have slight variations in color due to the presence of impurities. Anamosa limestone has a distinctive layered appearance (known as strata); each layer can vary in color based on what impurities are present in the rock. White/cream-colored layers are attributed to rocks with extremely high calcium carbonate content (little to no impurities). Tan, brown, orange, and rust-colored layers of limestone are due to the presence of traces of iron oxide in the rock. When limestone (containing iron oxide) becomes exposed to the elements, it changes color due to chemical weathering. Chemical weathering occurs when a rock is disintegrated by chemical reactions. When iron-rich rocks get chemically transformed into iron oxide, they get weakened and become more prone to erosion by water or wind.
St. Peter Sandstone: this type of sandstone is a sedimentary rock found in several midwestern states, including Iowa. 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. It consists of poorly cemented quartz sand grains, although there are trace amounts of feldspar. St. Peter sandstone, unlike Anamosa limestone, does not contain large visible layers.
Logging Tasks:
- Observe the six specimens of sedimentary rock at the GZ. What is the appearance and texture of the rock?
- Does the rock seem to be layered? If so, are there color differences (and corresponding impurities) between strata?
- Does the rock at the GZ appear to experience chemical weathering? Explain what you see.
- Make a final conclusion. Based on your observations at the GZ and what you’ve learned in the description, is the rock considered to be St. Peter sandstone or Anamosa limestone?
- As of June 2019, earthcaches now contain 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://www.iihr.uiowa.edu/igs/publications/uploads/EM-05.pdf
https://geokansas.ku.edu/limestone#:~:text=Pure%20limestone%20is%20white%20or,yellow%20to%20gray%20to%20red
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