Massive sandstone layers cap steep, barren, slope exposures of Mancos Shale along the east side of the Strike Valley. The Mancos Shale formed from marine sediments deposited in the Western Interior Seaway during the Late Cretaceous period. At its maximum extent about 100 million years ago, the Western Interior Seaway extended from the Arctic Ocean to the Gulf, and extended from the Cordilleran mountain ranges in the Nevada region eastward to the Mississippi Valley region. This seaway disappeared as the Laramide Orogeny uplifted the Rocky Mountain region beginning in the latest Cretaceous to the early Tertiary (about 50 million years ago). The region is still gradually rising today. The Mancos Shale can be subdivided into units from lowest to highest shown here: The Tununk Shale underlies the foreground valley;, the first ridge is the Ferron Sandstone Member; the second valley and gray slope is the Blue Gate Shale Member, and the caprock on the distant ridge is the Emery Sandstone Member; the uppermost Masuk Member (mostly shale) is not visible in this image.