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Faulted chert nodule, Rincon Mountains, near Tucson, Arizona


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Age & Formation: 
Horquilla Formation of Pennyslvanian age, ~310 Ma.
Geohistory: 
This chert nodule was originally embedded within limestone, but because of its comparative resistance to erosion completely weathered out of the bedrock. It is fascinating to see that the fractures cutting the chert nodule are actually faults with clear offset. The effect of the faulting was to lengthen the nodule in one direction (parallel to the length) and flatten it at right angles (perpendicular to the oblate spheroid).
Whereabouts: 
I collected this in the late 1970s in the Rincon Mountains in the Happy Valley area. It is one thing to find a small rock that is marked by a single fault, and quite another to find one with a system of faults. I like the fact that by eye it is ‘easy’ to ‘unfault’ the rock and get back to the original shape and size of the chert nodule.