Rock Type: Gray shale Formation: Allegheny Interval: 5-Block seam Age: Middle Pennsylvanian Period, approx. 303 million years. The pith cavity of Calamites is hollow. When shifting meanders of ancient deltas washed over stands of these prehistoric plants it opened these cavities which easily filled with the sandy sediments the currents were carrying. Therefore, the characteristic pattern we associate with Calamites is actually a pseudofossil, or non-organic cast of the hollow, inner pith cavity. 1. View a close-up image.