Geologist Mac Jervey stands on the Mosier Erratic, a 6’x8’x12’ quartz boulder at the Twin Tunnels trailhead west of Mosier. Contributed photo
Glacial erratics were transported in ice blocks that swept down from northernmost Idaho and Montana in the Missoula Floods between 18,000 and 15,000 years ago.
White Salmon resident Terry Hurd will address the geology of the Columbia River Gorge Saturday, Feb. 1, in the first of four Regional History Forum programs at the Original Wasco County Courthouse, 410 W. 2nd Place, The Dalles. The program entitled “Gorge Geology: Colliding Plates, Lava Flows, and the Missoula Flood,” begins at 1:30 p.m. The program takes place in the upstairs courtroom of the 1859 building. There is a TV monitor to the downstairs sheriff’s office to accommodate those unable to climb the stairs. Coffee and cookies will be served after the program.
Hurd is president of the Columbia River Gorge Chapter of the Ice Age Floods Institute. He is a retired fisheries biologist and amateur geologist. His PowerPoint program includes photos of nearby geologic features as well as the locations of glaciers and the ice dams that failed, resulting in catastrophic floods that forever altered our surroundings, scouring the valley with 500 cubic miles of water and massive blocks of ice.
Eighteen million years ago, the area of the gorge was a 40-mile-wide lowland. A collection of streams draining from the central Washington plateau joined and flowed west to the Pacific Ocean forming the ancestral Columbia River. About that time the North American tectonic plate was overlying a hot spot now underlying Yellowstone National Park. Over the next 12 million years or so, lava flows from deep fissures in the earth’s surface near the Washington-Idaho-Oregon border erupted and spread rapidly across thousands of square miles and to the Pacific Ocean. This event occurred over 300 times.
During the same time period the North American continent continued to slowly move to the southwest. Its collision with the Farallon and Juan de Fuca oceanic plates pressing in the opposite direction caused the continental crust to buckle and fracture. The resulting folds played a major role in the location of the Columbia River Gorge and the later glacial-outburst floods from Lake Missoula.
Between 18,000 and 15,000 years ago a lobe of the Cordilleran ice sheet repeatedly formed an ice dam in the Purcell Trench of northern Idaho. These 2500+ foot high dams blocked the mouth of the Clark Fork River backing up a 3000 sq. mi. and 2000 foot deep lake, Glacial Lake Missoula.
As the lake waters rose the eventually floated and hydraulically undermined the dam until it suddenly failed catastrophically, releasing 500 cubic miles of glacial melt water and massive blocks of glacial ice. These huge floods, about 100 in number, flowed at rates of ten times the combined flow of all of the current rivers of the world draining the lake in a matter of days.
The flood waters reached the Gorge in about ten days forever altering its features.