The Grand Coulee is an ancient river bed in the U.S. state of Washington. This National Natural Landmark stretches for about sixty miles southwest from Grand Coulee Dam to Soap Lake, being bisected by Dry Falls into the Upper and Lower Grand Coulee.
The Grand Coulee is part of the Columbia River Plateau. This area has underlying granite bedrock, formed deep in the Earth’s crust 40 to 60 Ma (million years ago). The land periodically uplifted and subsided over millions of years giving rise to some small mountains and, eventually, an inland sea.
From about 10 to 18 Million years ago, a series of volcanic eruptions from the Grand Ronde Rift, near the Idaho/Oregon/Washington border began to fill the inland sea with lava. In some places the volcanic basalt is 2,000 metres (6,600 ft) thick. In other areas granite from the earlier mountains is still exposed. Many animals roamed the area including camel, horse and rhinosaurus.
Two million years ago the Pleistocene epoch and glaciation. Large…
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