During the last Ice Age, 13,000-15,500 years ago, a giant glacier dammed the Clark Fork River in what is today southwest Montana and created Glacial Lake Missoula, a 3,000-square-mile lake holding nearly 500 cubic miles of water. As rising waters repeatedly collapsed the ice dam, tremendous floods swept across eastern Washington and down the Columbia River toward the Pacific Ocean, with waters nearly 1,000 feet deep surging through the Columbia River Gorge and flooding the Williamette Valley. Moving at 60 miles per hour and carrying 10 times the combined annual volume of all the Earth's rivers, these floods stripped away up to 200 feet of topsoil, tore huge boulders from bedrock, and carried boulder-laden icebergs that left stranded rocks called glacial erratics as the ice melted and waters receded. The 90-ton argillite boulder at the end of the quarter-mile trail was transported in this way and is the largest known glacial erratic among the hundreds found in the Williamette Valley. The Missoula Floods also deposited vast amounts of gravel, sand, silt, and clay that helped make the Williamette Valley one of the nation's most fertile agricultural regions, and soils formed in their wake are credited by enologists with the success of Pinot Noir, Pinot Blanc, and Pinot Gris grapes grown on some 40,000 acres there. Geologists estimate that this boulder originally weighed about 160 tons, and that more than 70 tons have been removed by visitors.

Photo: Duane Hall

Photo: Duane Hall

Photo: Duane Hall

Photo: Duane Hall

Amity, Oregon · USA