25 April 2011

Two joints

Posted by Callan Bentley

Stopped at Sideling Hill, Maryland, a few weeks back with my three Honors students, on our way to Pittsburgh for the northeast/north-central GSA section meeting. Robin took this photo of me with some sandstone beds that reveal two nice examples of joint anatomy, complementary in their structure:

First focus in on the area right of where my finger is pointing. It shows well-expressed plumose structure, little lines that point in the joint propagation direction, branching out from a point of origin, and exhibiting an overall feather-like morphology. Nice also the three decently-expressed concentric ribs, perpendicular to the plume hackles:

Above that is another joint face, and this one doesn’t show the plume hackles well, but has an excellent series of concentric ribs, which are arrest lines as the joint propagated from left to right. Looks like this particular face shows thirteen or so temporary stops along the way from unbroken to broken:

I like how you can see bedding expressed in the shadowed overhang of these ribs, too.