>>2343416My fossils are gathered in the Glen Rose formation, which is part of the Fredericksburg Group, in Texas. It is colored brown on this map.
The rock was formed around 95 million years ago during the mid Cretaceous. During that time, the entire middle third of the United States was covered by a shallow sea called the Western Interior Seaway.
The fossils I have found so far have been almost universally bivalves, as the other anon correctly identified. I have also found gastropods here, and I have heard many reports of ammonites.
The fossil is known locally as a Deer's Heart, although it was actually a type of clam in life.
