The Great Plains, as we have seen, is many things. It contains thick
layers of rock that formed in oceans, and younger layers of rocks
deposited by streams. These rocks have been affected by earth movements
and injected by hot molten rock, some of which reached the surface as
volcanic rock. The rocks have been carved by streams, dissolved by
ground water, partly covered by glaciers, and blown by winds. All of
these agents have played important roles in determining the landscape
and the landforms of the Great Plains. But the streams were the master
agent. They formed the great depositional plain that was to become the
Great Plains, and then began to destroy it—leaving only the High Plains
to remind us of what it was. Those long miles we travel across the High
Plains are a journey through history—geologic history.