During the Jurassic Period, southeastern Colorado was dominated by a vast but shallow inland lake. Along its muddy shore, in a gently rolling landscape of conifer forests laced with stream valleys before the Rocky Mountains had emerged, dinosaurs left footprints that are preserved with ripple marks, mud cracks, and casts of salt crystals. These rock layers formed along the edge of a brackish freshwater lake similar to today's Great Salt Lake, and they have also yielded freshwater clams, fish scales, plant impressions, and microscopic aquatic invertebrates that reveal the Jurassic world.