Those who travel between Denver and Colorado Springs are familiar with the scenic, mesa and valley terrain, north of the Palmer Divide. Contrasting with the stark wall of the Front Range foothills, this territory looks like it belongs in Western Colorado, where the rim of the Colorado Plateau curves through the State.
The geologic history of this topography dates back to the Oligocene, about 34 million years ago, when the ancestral South Platte, then a massive, braided river, flowed eastward out of the young Rocky Mountains. At that time, the mountains were nearly buried in their own erosional debris and recent volcanism (near present-day Salida) had spread an apron of ash across much of the region. The ancestral river, carrying rocks, pebbles and sand from the mountains, mixed and welded these fragments into a hard, conglomerate rock, the matrix of which contains the volcanic ash.
Further mountain uplift during the Miocene-Pliocene, combined with increased precipitation during the Pleistocene, altered the course of the South Platte River and augmented the erosive force of its tributaries; the latter include Plum Creek and Cherry Creek, which rise along the Palmer Divide and flow northward to join the South Platte. These two streams and their tributaries have eroded the mesas (protected by hard caps of Castle Rock conglomerate) from the surrounding plains, leaving the scenic landscape that we find today.
