Sometime between 5 and 10 million years ago, however, a great change
took place, apparently as a result of regional uplift of the entire
western part of the continent. While before, the streams had been
depositing sediment on the plains for more than 60 million years,
building up a huge thickness of sedimentary rock layers, now the streams
were forced to cut down into and excavate the sediments they had
formerly deposited. As uplift continued—and it may still be
continuing—the streams cut deeper and deeper into the layered stack and
developed tributary systems that excavated broad areas. High divides
were left between streams in some places, and broad plateaus were formed
and remain in other places. The great central area was essentially
untouched by erosion and remained standing above the dissected areas
surrounding it as the escarpment-rimmed plateau that is the High Plains.
This downcutting and excavation by streams, then, which began between 5
and 10 million years ago, roughed out the landscape of the Great Plains
and created the sections we call the Missouri Plateau, the Colorado
Piedmont, the Pecos Valley, the Edwards Plateau, and the Plains Border
Section. Nearly all the individual landforms that now attract the eye
have been created by geologic processes during the last 2 million years.
It truly is a young landscape.