D1 Synorogenic Strata This layer is composed of approximately 1,500 feet of alternating layers of sandstone from ancient river channels and mudstone from ancient flood plains. These layers were deposited during the early part of the uplift of the Front Range by rivers similar to the modern Platte River in northeastern Colorado. D1 Synorogenic Strata are coarse-grained along the Front Range and become fine-grained and coal-like to the east. These sediments were deposited in the Cretaceous Period and Paleocene Epoch (68-56 m.y.a). Typical fossils include petrified wood, leaves, dinosaur bones, and mammal teeth.