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