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.