| Summary: | This aerial view of the dam site was taken on April 21, 1940. This scene may give a clearer understanding of how the Grand Coulee canyon was carved in the great lava bed when the ice sheet blocked the course of the Columbia River a few miles downstream from the dam. In the distance looms Steamboat Rock, 2 miles long and 900 feet high, which once stood as a Goat Island as the immense falls of the glacial flood retreated toward the river canyon. Remembering that this coulee is cut thru lava beds more than a thousand feet deep, and that these flows covered hundreds of thousands of square miles, some idea of the magnitude of this enormous outpouring of molten magma may be gained from this picture. A view meant to illustrate the geological events that formed the Columbia River Basin.
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