Leaving Denver this morning, I headed east across the High Plains of eastern Colorado which have a veneer of late Tertiary sediments (about 5-30 million years old), eroded from the Rockies or blown eastward from volcanoes of the Western U.S. Now sculpted into broad uplands and valleys across the Colorado Piedmont, these deposits form loosely compacted strata that overly Cretaceous shale and sandstone; the latter sediments, some 100 million years old, are upturned as a hogback along the base of the Front Range and are exposed across the "lowlands" of the South Platte Valley.
By the time I reached central Kansas, the relatively young Tertiary sediments had disappeared and Cretaceous limestones lay just beneath the soil; these chalks and "post-rocks" were deposited in a broad, shallow sea, which stretched from the Texas coast to northwest Canada about 100 million years ago (MYA). Just east of Junction City, I-70 climbs into the Flint Hills, composed of Permian strata (about 250 million years old); these scenic ridges are the northern end a a Permian swath that stretches southwestward to the Permian Basin of West Texas. It was during the Permian Period that Earth's continents merged to form Pangea, forcing up the Southern Appalachians as North American collided with Africa.
In easternmost Kansas and western Missouri, Pennsylvanian rocks are exposed along rivers and roadcuts; deposited about 300 MYA, they represent a time when fern forests dominated the globe, home to early reptiles and giant amphibians. Nearing Columbia, outcrops of Mississipian limestone appear along the highway; deposited in shallow seas, some 325 MYA, this bedrock now yields the karst topography of gorges, caves, springs and sinkholes that characterize Central Missouri. Over the course of 11 hours, I covered 700 miles and traveled back through 300 million years of geologic history.
