I collected this rock in the region of the Coyote Mountains in southeastern Arizona in ~1976, while carrying out regional structural geologic mapping of metamorphic core complexes. What I especially like about this specimen is that is in the form of a block (paralllelpiped) with wonderful planar, perpendicular edges defined by joint surfaces. Thus every surface is structural, and not just fashioned by erosion and weathering. Two of the surfaces are foliation/lineation surfaces. Four of the surfaces are joint surfaces. Some joint fringes are evident on the shoulders of the joint surfaces.
Age & Formation
Cambrian Bolsa Quartzite, ~~520 Ma. It was transformed into mylonite in the Miocene, during ~25-18 Ma.