After taking pictures of Trimble Knob last Saturday, we headed east on US 250, crossing several mountains that have lots of roadcuts through lower Devonian sediments. One of these exposures has about 50 vertical feet of rock that epitomizes “crinoidal limestone”:
This is pretty amazing rock. Almost everything in the picture is a crinoid fragment, mostly stem segments. There were smaller numbers of brachiopods and bryozoans, also fragmented, although I don’t think there are any in this picture. There were also some larger structures:
Deposits like this are death assemblages, which represent the transport and accumulation of skeletal debris that doesn’t necessarily reflect the living ecosystem. The fossil fragments are acting as sedimentary grains (like sand grains). Other than the possible corals (which might be in place), the fossil fragments are all about the same size (in other words, it’s well-sorted). There is even cross-bedding apparent in the distribution of the fragments, showing that the fossil bits were mounded up into ripple marks by the current:
Unfortunately, the geologic maps through this area are a little vague as to exactly which unit this is; most of the maps list this spot as “Devonian-Silurian, undifferentiated”. Based on the descriptions of the various units in the Geologic Map of Virginia, I believe this is either the upper member of the Keyser Formation or the overlying New Creek Limestone, which are both lower Devonian limestones that are crinoidal in places.