This rock was identified at the whatsthisrock subreddit as a "penecontemporaneous deformation structure." It apparently is such a commonly-used term that it is shortened by users to "PCD." There is excellent informed discussion at the link to explain that PCDs are formed when sedimentary material is deformed during deposition ("contemporaneously"). Lots of further details at Geological Digressions.
I thought the rock was cool, but what grabbed my attention was the fact that I am an English major almost 80 years of age and I'm seeing a prefix that is not in my wheelhouse.
Onward to the Wiktionary entry for "pene":
Almost the thing or quality expressed by the root, as peneplain (almost a plain), peninsula (almost an island), penultimate (almost the last), penumbra (almost in shadow).
Wow. Three words I've used for essentially all my adult life without appreciating their common prefix.
You learn something every day.
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