Friday Rocks #21: Miniture Faults in Sandstone

Photo by Tim Sherry

I’ve been posting a lot of Garden of the Gods pictures lately. We had a great field trip led by Christine Siddoway and Elisa Fitz Díaz. Here is a fault zone with centimeter offsets. Try unfaulting the photo to see what the rock looked like before it was faulted. How much apparent offset is there on each strand?

Friday Rocks #20: Boudinaged Obsidian

Obsidian is a volcanic glass, lava that has cooled so quickly that crystals don’t form. In Panama Crater, Mono Lake, California this lava was partially cooled. Stresses pulled the partially solidified obsidian apart, like taffy, forming what Structural Geologists call “boudinage”. The less cohesive rock surrounding the boudining obsidian filled the gap. Other obsidian layers above and below the boudined layer have “bent” and deformed towards the boudin.

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