In the history of the earth, the Western Interior Seaway was a shallow marine waterway that covered about 1/3 of what is not the Western US. It connected the Gulf of Mexico to the Arctic Sea. It range from what is now Kansas West to what the western Nevada. Yes the Rocky mountains used to be under a sea. Because of that layers of laid down on the sea floor. These layers were formed of shale and sandstone. The uplift that created the Rocky mountains was from about 55-85 million years ago. Following that the uplift further west as later which formed much of the intermountain region in Western Colorado, Utah and into the great basin of Nevada. The San Rafael lifted the sea bed which was the sea bottom of layers.
I would have liked to have had more time to spend. We might go back after my wife gets her new hip and I get my new knees and are more mobile. It is an area worth exploring and more importantly it has not been discovered by the "horde."