bedrock. The Sonoma orogeny uplifted the ancestral Rocky Mountains around 300 million years ago, which reached heights of around 10,000 feet. The Laurentian continent was colliding with the supercontinent Gondwana to the south, to form the larger supercontinent Pangaea. This continental collision generated two large island mountain ranges, the Frontrangia and Uncompahgria. Erosion quickly wore away at the mountains, shedding sediments across the landscape. Mesozoic (251-66 million years ago) Dark red Triassic shale and wind-blown sands transformed into sandstone overlie Permian rocks, marking the beginning of the Mesozoic. The sea reached western Colorado and the area was blanketed in ash fall from eruptions to the west. Early Jurassic dune sands were covered over by the sandstone and shale of the Morrison Formation, until a large scale marine transgression flooded the region in the Cretaceous. As sea levels fell, the Dakota Formation sands and pebbles took shape, followed by another marine transgression that deposited several thousand feet of shale as the Mancos Formation and Pierre Formation. Intermediate water depths filled with the Niobrara limestone. Sand layers point to periodic regression and shale layers contain extensive marine fossils. With the beginning of the Laramide orogeny that uplifted the Rocky Mountains, coastal wetlands formed large coal deposits. Thick coal and sandstone layers that grow thicker to the west are known as the Mesaverde Group. Cenozoic (66 million years ago-present) The Laramide orogeny continued in the Cenozoic up until the Eocene with tectonic forces shifting from north–south mountain ranges to east–west. The Uinta Range is unique, trending east–west due to older tectonic constraints. In the Denver Basin, the Denver Formation and Dawson Formation formed in the Paleocene from sand and gravel eroded out of the Rockies. To the west, rivers filled the lowlands with the Wastach Formation. Erosion and uplift occurred in sync, creating mountains that tended to have gradual slopes. After the end of uplift 20 million years ago, erosion cut down to bedrock in the mountains. In the northwest, the Green River Formation siltstones formed as a lake deposit in a basin formed along Proterozoic faults and was covered over with siltstone and sandstone of the Uinta Formation. Together with Utah and Nevada, Farallon Plate related volcanism in the Oligocene produced enormous eruptions that built up the San Juan Mountains in the southwest. One third of the region was covered in ash flow. Throughout the Miocene, large scale regional uplift elevated Colorado,