The Bone Valley Formation is what geologists have dubbed the ancient fossil bed that now resides 20 to 40 feet below Central Florida. It sits atop the Hawthorne Formation, a layer of clay and sandy limestone deposited long ago when the sea covered the state and the coastline curved around the modern-day Lake Wales Ridge as much as 60 miles inland. The Bone Valley Formation is exposed in the phosphate mines of central Florida (Polk County). It is Miocene in age (10-15 million years ago).