Grand Coulee is an ancient river bed in the U.S. state of Washington. This National Natural Landmark stretches for about 60 miles (100 km) southwest from Grand Coulee Dam to Soap Lake, being bisected by Dry Falls into the Upper and Lower Grand Coulee. Geological history Grand Coulee is a large coulee on the Columbia River Plateau. This area has underlying granite bedrock, formed deep in the Earth's crust 40 to 60 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/Montana border began to fill the inland sea with lava. In some places the volcanic basalt is thick. In other areas granite from the earlier mountains is still exposed. Starting about two million years ago, during the Pleistocene epoch, glaciation took place in the area. Large parts of northern North America were repeatedly covered with glacial ice sheets, at times reaching over in thickness. Periodic climate changes resulted in corresponding advances and retreats of ice. About 18,000 years ago a large finger of ice advanced into present-day Idaho, forming an ice dam known as the Purcell lobe at what is now Lake Pend Oreille. The Purcell lobe blocked the Clark Fork River drainage, thus creating an enormous lake reaching far back into mountain valleys of western Montana. Leaks may have developed around and under the ice, causing the dam to fail. The of water in Lake Missoula were released in just 48 hours—a torrential flood equivalent to ten times the combined flow of all the rivers in the world. This mass of water and ice, high near the ice dam before release, flowed across the Columbia Basin, moving at speeds of up to . The deluge stripped away soil, cut deep canyons and carved out of earth, leaving behind areas of stark scabland. Over nearly 2500 years the cycle was repeated many times. Most of the displaced soil created new landforms, but some was carried far out into the Pacific Ocean. In Oregon's Willamette Valley, as far south as Eugene, the cataclysmic flood waters deposited fertile soil and icebergs left numerous boulders from as far away as Montana and Canada. At present day Portland, the water measured deep. A canyon deep is carved into