Friday, August 1, 2014

Friday Rocks #22: Tonalite Mega-Boudin

The large lensoidal feature in the middle of the photo is a Tonalite sill Mega-Boudin surrounded by schist.

Friday, July 25, 2014

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, July 18, 2014

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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Friday, July 4, 2014

Friday Rocks #19: Ammonite

Here is a Ammonite fossil in cretaceous sandstone outside of Manitou Springs, CO

Photo by Tim Sherry 

Friday, June 27, 2014

Friday Rocks #18: Pencil Cleavage

Pencil Cleavage is developed through the intersection of cleavage and bedding planes. The fractures form long, skinny pieces of rock.

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