Notice the distinct layering of the rock.  This site in Montana shows a separation point between the age of dinosaurs and later periods.  Paleontologists and evolutionary biologists take advantage of areas of extensive erosion where layers of ancient rock have been exposed.
 

Red sedimentary layers in Wyoming mark the first known North American occurrence of several mammal groups, some 55 million years ago.  Fossils found in these layers reveal extinct dog-size like predators, called hyaenodontids.