The I-90 ash-filled paleochannel

Driving west on I-90 near the Garden Springs exit, number 277A, you can see more and more basalt...and then a short exposure of white layers in the rock overlying gravel. These layers are mostly ash, or tephra, from highly explosive volcanoes. When the volcano erupts violently the magma cools and fragments as trapped gases expand, resulting in ash. Wind can transport ash for miles until it finally settles. The ash observed in the photo appears to deposited by multiple volcanoes ranging back to around roughly 13,000 years ago. These deposits fill an old channel that appears to have flowed to the south, since another cross-section of the channel shows up on the other side of the road too.

The first (lowest) layer of angular blocks of basalt are possibly deposited by the Missoula Floods. The ash layer overlying the gravels is possibly from Glacier Peak in the northern Cascade Range of Washington (based on Stradling and Kiver, 1886). This first layer of ash may be approximately 13,000 years old. However, further identification of the ash is needed as many glacial outburst flood deposits in the are may be younger than the Glacier Peak B amd G layers. The next and thickest layer of ash appears to be the climatic Mazama ash, from when Crater Lake lake formed roughly 7,600 years ago, the ash in this part of the deposit contains ferromagnesian minerals and lithic fragments. Finally, the top layers we were likely from Mt. St. Helens. If so, this deposit is estimated to be less than 3,900 years old. This Mt. St. Helens ash is from the spirit lake stage of its history. The majority, of this ash, contains tephra and andesite.

The layers within this ash deposit is comprised primarily of tephra, dacite, andesite, and rhyolite. This large layer is formed by multiple geologic events that have taken place over thousands of years. It’s amazing that this simple looking, white layer in the rock can provide so much history of the surrounding areas of eastern Washington.


Picture of the ash layers exposed in road near the Garden Springs exit off I-90, just west of Spokane

Picture of the ash layers exposed in road near the Garden Springs exit off I-90, just west of Spokane

During construction of I-90 this cross-section of an old channel was exposed. These are the expected ash layers, based upon conversations with local geologists and WSU's Nick Foit, the tephra king:) | Source: Bradley Thompson | Creator: Amie Andersen & Flood Explorer Team View File Details Page

General Eruptive History of Mt St Helens

General Eruptive History of Mt St Helens

Source: Clynne, M.A., Calvert, A.T., Wolfe, E.W., Evarts, R.C., Fleck, R.J., and Lanphere, M.A., 2008, The Pleistocene eruptive history of Mount St. Helens, Washington, from 300,000 to 12,800 years before present: U.S.Geological Survey Professional Paper 1750, p. 593-627. View File Details Page

Cite this Page:

Amie Andersen, “The I-90 ash-filled paleochannel,” Ice Age Floods Explorer, accessed October 22, 2017,


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