This specimen is a sandstone cast of Calamites cisti-- an ancient relative of modern horsetails. It is a cast of the inner pith cavity and shows the vertical ribs and horizontal rows of "nodes" which are characteristic of the inside structure of calamites.
Typically, calamite stems are preserved best when they are buried by sand while standing, whereby the relatively hollow stem is broken at some stage, allowing sand in-filling. Calamite stem casts in shale are generally less well preserved.
In general, the best fossil assembledges of calamite stems and foliage occur when they are buried during deltaic flood events which deliver sands initially with relatively higher stream energy that cover and preserve standing stems, followed by deposition of clays and fine silt deposited at very low stream energy that preserve the leaves. In fact, calamites appear to have grown predominantly next to open areas, like river banks-- presumably because they did not grow as tall as lycopods like Lepidodendron or Sigillaria, and they could best compete for sunlight at such locations.
This is one example of how geologists use such clues to interpret the "depositional environment" of rocks by the fossils they contain.
Location: Nicholas County, West Virginia; Alex Energy. Inc., Robinson
North Surface Mine, located about 10 miles west of Summersville, West Virginia,
just north of Route 39. It is an active surface mine and requires permission