The sheer cliff here was once the world's greatest waterfall, an Ice Age floods feature whose western portion remains visible. When the last Ice Age ended, floodwaters no longer swept through Grand Coulee, and the waterfall was left high and dry. As raging floodwaters had poured over the lip of the falls, they plucked chunks of basalt from its base, causing overhanging cliffs to collapse into the plunge pool and pushing the falls farther upstream with each collapse. Over time, the waterfall moved 18 miles from Soap Lake to Dry Falls, creating Lower Grand Coulee, and the force of water dropping hundreds of feet over the falls eroded deep hollow spots below it called plunge pools.