After racing across eastern Washington, floodwaters converged on Wallula Gap, a single narrow outlet about 80 miles to the southeast. This natural constriction forced the water to back up and create a huge temporary lake called Lake Lewis. At this viewpoint, the lake would have stood more than 500 feet higher during the largest floods, and in less than a week the huge lake was gone. When the floodwaters drained away, hundreds of icebergs carrying rocks from as far away as Canada were stranded. As the ice melted, these exotic boulders, called erratics, were left behind. During the largest floods, much of the surrounding countryside was under water. Trapped in ice, erratics traveled hundreds of miles until the ice ran aground and melted, and they are most common in areas where floodwaters backed up and formed temporary lakes. Boulders called erratics were left at Ginkgo, some stranded more than 700 feet above the river, and most are made of granite and diorite.