Scallop shell impression on a small rounded stone. If discovered in a river bed or along shore lines in the Great Lakes Region of the United States; this is possibly an impression from the Silurian Period. The Silurian period took place more than 400 million years ago and was a time of glacial melt. The rock appears to be granite which is a holocrystalline rock mainly consisting of feldspar, quartz, mica, and amphibole minerals. Unknown dimensions; Unknown weight.