23 March 2012
Hand sample of folded limestone strata in West Virginia (presumably Devonian in age). Note the rip-up clasts and large grain size at the base of the sample (to the right in the photo). Note the fine-grained, thin-bedded shale laminations towards the top (left) of the sample, too. Together, they tell a story of decreasing energy in the environment of deposition: maybe an ancient storm deposit (a “tempestite,” one of the best words in all of geology) and its aftermath. And then, of course, the whole thing got folded, during Alleghanian mountain-building (the collision of ancient Africa and ancient North America that made Pangea). The photo’s perspective is roughly down the axis of the main fold. You can also see some accompanying parasitic folds on the top (left) in the shale layers whose limbs are rotating into parallel, developing into a decent crenulation cleavage. Because of the coarser grain size and lack of clay, the lower portion (right side) of the sample displays no evidence of any incipient cleaving.