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As the Interior Seaway retreated, its western coastline changed from a marine environment to coastal swamps, estuaries, and delta plains. Fox Hills Sandstone is the relict of this transition. The base of the Fox Hills rock unit fades into marine shales, whereas in most transitional rock units the shift from coastline sandstone to back-beach, mud-rich deposits is very abrupt.
In Colorado, the Fox Hills Sandstone is comprised of a series of discrete sandstone beds that are typically 6–12 miles wide, 30–50 feet thick, and are arranged in a series of off-lapping shingles of sandstone.
On Rooney Road, immediately south of Interstate 70, you can stop to admire an outcrop of Fox Hills Sandstone. Notice how the Pierre Shale of the Interior Seaway grades gradually into thicker and thicker beds of this brilliant white rock. Take off your shoes and walk on the beach!
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