"In the late Cretaceous period, from 90 to 65 million years ago, Montana had a lusher climate than today. The Rocky Mountains formed one edge of a vast inland sea - Fort Peck was beachfront property on the edge of that sea. There are three distinct sedimentary rock formations from that era running through the area. The T. rex, "Peck's Rex," was found in the Hell Creek formation in 1997, just inland from the ancient coast. The sparsely-fossilized Fox Hill sandstone is a remnant of the beach itself. And the third formation, known as the Bear Paw Shale, was deposited under the warm, shallow waters.
The Bear Paw Shale is a bit of an anomaly. Oh, the fossils are there, but instead of being layered right into the rock, many of them are found inside stony nodules similar to geodes. These nodules, which range in size from a few centimeters to more than a meter in diameter, may contain any sort of pre-historic life from ammonites to turtle shells, often with delicate details - a tiny leg, a thin layer of mother-of-pearl - preserved."