During the Jurassic, the Colorado landscape looked quite different than it does today. The climate was moist and teeming with vegetation -- possibly like today's Amazon Valley.
The soft sandstones and interbedded mudstones that characterize the Morrison were deposited in shallow lakes and by streams and rivers that would occasionally escape their banks and flood the surrounding lowlands. Fossilized remains of dinosaurs and other Mesozoic life provide important clues which help us understand the environment and the creatures that lived near these ancient lakes and streams.
Above the Morrison is a more resistant sedimentary rock called the Dakota Sandstone. This buff-colored, Cretaceous rock forms prominent outcrops in the roadcuts along Highway 50 just west of Elk Creek.
The Dakota Sandstone was deposited along the shore of an ancient inland sea. Imagine a coastal area with beaches, sand dunes, marshes, and mudflats -- that is what this area looked like when the Dakota was being deposited.
Over the next several thousand years, the inland sea deepened and expanded, swallowing Colorado and much of the Southwest. Near-shore beach deposits gave way to offshore marine deposits of soft, black mud. This mud accumulated to great thicknesses. Over time, the mud turned to soft rock and is now called the Mancos Shale.
The Mancos Shale is named for the town of Mancos, Colorado, located just east of Mesa Verde National Park. This soft, dull, grey formation often alters to a yellowish hue and forms smooth slopes or rounded mounds where exposed. When wet, the Mancos can be quite the trouble-maker, quickly transforming into a thick, glue-like, soupy mess. It absorbs water like a sponge and is highly prone to landsliding, an unfortunate fact since it is found throughout western Colorado and eastern Utah.
As you drive along Highway 50, look for evidence of landslides between Blue Creek Canyon and Montrose. The "hummocky" topography illustrates the dangers of building roads or buildings on the unstable Mancos Shale!