Thursday, July 17, 2014

Front Range Geology

During the Cretaceous Period, 135 to 65 million years ago (MYA), a shallow sea stretched from the Texas Gulf Coast to British Columbia, covering much of Colorado; deposits within and along this sea would yield the Dakota sandstone and Pierre shale that are now evident near the base of the Front Range and across the Colorado Piedmont.  Near the end of the Period, a layer cake of horizontal Mesozoic and Paleozoic sediments stretched above the deep Precambrian basement rock (youngest to oldest from top to bottom).

Then, about 70 MYA, the Laramide Orogeny began and the Front Range of the Rockies crumpled skyward, reinforced by a second uplift during the Miocene Period, some 25 MYA.  The overlying strata were tilted upward, toward the crest of the range, and erosion began to sculpt the layers; resistant sheets of sandstone produced fins and ridges while intervening, softer layers of shale eroded into valleys.  Meanwhile, Tertiary debris, eroded from the Front Range, was carried eastward by many streams and rivers that dropped from the mountains and meandered across the High Plains; as a result, a veneer of these younger deposits covered the Cretaceous shale and sandstones left behind by the retreating sea.

Today, driving westward toward the Front Range, one crosses the Tertiary landscape of the High Plains, broken by valleys of Pierre shale and Cretaceous sandstones along the South Platte River and its tributaries.  At the foot of the Rockies, one encounters the Dakota hogback (Cretaceous sandstone), backed by the Morrison Formation, a valley of Jurassic shale; both are rich in dinosaur fossils.  West of the Morrison valley is an outcrop of Permian sandstone (the yellow-gray Lyons Formation) and, up against the foothills, are the salmon-colored sandstone fins of the Fountain Formation (Pennsylvanian in age).  The foothills and mountains are uplifts of ancient Precambrian granite, laced with Tertiary and Quaternary debris along their many streams.  In effect, the traveler crosses progressively older rock strata from the High Plains to the Front Range.