Before you is the work Sacramento Stele created by Beverly Pepper. It features 4 large stone columns each stretching 18 feet high. Inscribed unto the surface is the poem “Also Blossoming” by Jorie Graham.
The columns are made of pietra serena stone, which is a blue-gray sandstone that hails all the way from Italy. It became widely popular in the Italian Renaissance.
The rock has a natural blue-gray color, yet often turns to a lighter reddish brown after extraction from quarries. This is due to the clay within the stone undergoing a process called oxidation. Oxidation is a chemical reaction occurring when a rock or mineral comes into contact with oxygen and/or water. This can lead to changes in its chemical composition, which is most noticeable in the form of a color change!
Pietra serena comes from the Macigno Formation, which is a turbidite. A turbidite is sediment that has been deposited as a result of turbidity flows, which can be pictured as an underwater landslide due to seismic activity (earthquakes) or gravitational forces (collapsing slopes, etc.). The deposition of sand in these underwater landslides laid the sentiment down for pietra serena stone, and pressure over time solidified it into rock.
The pietra serena is a specific layer of sedimentary sandstone within the turbidite of the Macigno Formation. This layer is classified as having the most sand content, also known as an arenaceous level. The grain size of pietra serena is medium to coarse. Pietra serena also contains varying levels of calcite, which is a variable in its durability and hardness. With more calcite cement present in the stone, it will resist weathering and withstand much harsher conditions.
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1. The name of this earthcache and the number of people in your group.
2. Observe the stone columns. Although on first glance the stone looks light gray/brown, take a closer look. Do you see any traces of blue-gray? Why or why not?
3. Looking very closely, observe and describe the grain size of particles within the stone columns. Feel free to run your hands along the columns.
4. Based on the use of pietra serena in this capacity, infer what this means about this sample of stone’s calcite content (consider that some pietra serena is also used indoors, where weathering is not a factor).
Sources:
-https://en.wikipedia.org/wiki/Pietra_serena
-https://publicartarchive.org/art/Sacramento-Stele/c349554d
-https://www.stonemania.co.uk/blog/what-is-oxidation
-https://www.sciencedirect.com/science/article/abs/pii/S1296207408000307