That's Bluestone (actually a type of sandstone) which Ulster County is famous for. It was and still is very popular as a building material and large quantities are still quarried for that purpose. The stone was formed in the upper Devonian period (360-385 million years ago) and is often quite fossiliferous. During the Devonian period life was just starting to colonize dry land and mostly consisted of mosses, early vascular plants, and arthropods (bugs). It's also generally believed that fish first started to move to dry land during this time. However the shape is just an interesting coincidence since nothing at that time would be leaving footprints like that and the shells strongly suggest that this was a marine deposit anyways. It's a nice specimen though with a great shell density; worthy of any mantle!
