More than 100 million years ago during Cretaceous time, tan sandstone of the Dakota Group was deposited along the western shore of the Western Interior Seaway and now forms the crest of Dinosaur Ridge. At that time, only low hills existed to the west because the present Rocky Mountains had not yet formed. At its fullest extent, the Seaway stretched from the Gulf of Mexico to Canada. As it grew and receded, it left sediments characteristic of beaches, swamps, and coastal plain wetlands, and the plants and animals whose traces and tracks are preserved in the rocks today lived in those ancient nearshore and intertidal environments.