Here is a Green River Knightia eoceana fossil from my collection that comes from the early Eocene period around 50 million years ago. It's closest living relative is the Alewife, a small fish I'm very familiar with because they invaded the Great Lakes region beginning in the 1950's and used to wash up by the thousands on the beaches of Lake Huron and Lake Michigan in massive die-offs after their population exploded due to lack of predators.
The thing I find most interesting about the Green River Formation located in western Colorado, eastern Utah, and southwestern Wyoming besides the incredible array of high quality fossils found there is that the fossils are often found in very distinct annual layers called varves. Darker colored sediments were laid down during the summer season as vegetation bloomed and lighter colored sediment was laid down during the winter. There are literally millions of varves (layers) in some areas. Geologists see this happening in lakes today so it makes great observable evidence that demonstrates it took millions of years for these extremely fossiliferous layers to form.
Soft fine grained sediment deposited in millions of thin annual layers makes the rock easy to separate so we can see the fine detailed evidence locked inside. A fossil hunter's dream. Evidence that no worldwide flood was needed for the formation of millions of fossils. Evidence the Earth cannot be only 6,000 years old. Evidence a literal interpretation of Genesis is wrong.