Wenatchee Washington – Ice Age Floods Geology

Hello young people. The Ice Age Floods at Wenatchee, Washington. The Columbia River and the foothills of the Cascades. There’s a great Ice Age Floods geology story here. And the Columbia River is part of it. Mainly deposits though, we’re not talking about ripping up the ground by the floods here. We’re talking about depositing material out of the Ice Age floodwater. Upstream from Wenatchee, the Okanogan Glacier an ice-sheet came down from Canada and blocked the Columbia.

Sending water into Grand Coulee and over Dry Falls and protecting Wenatchee from the power of these Ice Age Floods. But there were times during the Ice Age that the Okanogan was not there and the Ice Age Floods could come all the way down the Columbia River. Around the horn and down over the city of Wenatchee. 65 Mile an hour water, 1,000 feet deep! There’s more to the story than that. The Columbia Valley is narrow upstream from Weanatchee cut into stubborn metamorphic rock called gneiss. There’s also a narrow Columbia Valley downstream from Wenatchee, cut into basalt. But here at Wenatchee the basin is open it’s broad, so the Ice Age floodwater coming down the Columbia slowed here, filled this basin and waited their turn to exit south through the narrow canyon. That means that we had water creeping up neighboring valleys, as well as depositing truly giant flood bars.

The most famous of which is Pangborn Bar, deposited on the inside curve of the Ice Age Floods. A pile of rock 600 feet high. The high-water mark above Pangborn Bar, 1,700 feet in elevation that means we had so much water here that the water started to slowly creep up to the west, the Wenatchee River Valley. Quiet slack-water over Cashmere, Dryden, Peshastin. Ice Age floodwater getting all the way up into the Cascades as far as Leavenworth. Have you heard this before? Heck, there’s ice at this time up at Leavenworth coming out of the Icicle drainage. And yet, we’ve got Ice Age floodwater lapping up against that glacier. That’s quite a story, what evidence do we have for that? We’ve used Ice Age flood erratics and rhythmites to reconstruct this part of the story. Erratics, light-colored boulders scattered on the hillsides here in the Wenatchee River valley. Up to 300 feet above the river are those boulders, up to 600 feet above the river here at Wenatchee, we’ve got these ice-rafted erratics. Telling us where the high-water mark was of this quiet bathtub water working it’s way up the Wenatchee River.

In fact, all the beautiful orchards that are in the floor of the Wenatchee Valley are on Ice Age Flood deposits. Those are the rhythmites, fine-grained chalky sedimentary materiel deposited at the bottom of this Ice Age water. Each rhythmite talking about an individual flood event. And maybe not all of the water surging up the Wenatchee River Valley is coming from the north. Moses Coulee is not very far away. And there was a big broad giant flood bar that sealed off the mouth of Moses Coulee that trapped water and sent more slack-water upstream and into the Wenatchee area. The Ice Age Floods at Wenatchee, Washington.


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