The Ellensburg Formation: A Distant Cousin to Kitsap's Blakely Formation.

It's been almost a year to the day since I've last posted. I hope to get a flow going again, especially since I'm graduating this March. So here we go.

The Cascade volcanoes have been erupting for 40 million years, constructing massive volcanic edifices, ejecting clouds of debris and ash to rain down on eastern Washington, and sending mudflows and floods careening down river valleys into the Pacific Ocean, Puget Sound, and eastern lowlands. Today with the current position of our five active fire mountains, most, if not all, lahars are directed to the west towards Puget Sound with its densely populated metropolitan centers or south towards the Columbia River. Millions of years ago, the setting was slightly different. A volcano or a group of volcanoes near the present-day location of White Pass were active, violently active. Explosive eruptions blasted frothy gas-rich magma out of their throats and sent moving masses of water, ash, and clay barrelling down ancient river valleys. One of these valleys was aimed at the present-day location of Thorp, Washington.

The Kittitas Valley with its modern-day volcanic sentinels Mount Adams (Left) and Mount Rainier (right). 

Driving east along Interstate 90 past Thorp, one can look across the valley to the north and spot a group of imposing white cliffs towering over the adjacent fields. For a closer look, Highway 10 between Ellensburg and Thorp passes immediately below these cliffs. These are the "White Bluffs," and they are a geologic staple of the Kittitas Valley. These cliffs reveal a stack of 4-5 volcanic lahars produced by the volcanoes near White Pass so many millennia ago. The lahars appear as a solid white mass of ash-rich cement, suspending huge boulders of volcanic rock and peppered with sponge-like pumice, which is typically produced in significantly explosive eruptions. Each lahar is separated by beautifully layered river deposits, organized and sorted beds with crossbeds and other patterns, marking periods of slower processes compared to the catastrophic volcanic apocalypses which ravaged the river valley half a dozen times. 

Outcrop of the "White Bluffs" showing a lahar (Thick layer at the base) and river deposits over the top. 

This outcrop is just a small section of the Ellensburg Formation. This geologic formation can be found throughout the Kittitas and Yakima Valleys and varies significantly throughout its extent. Just southwest of Central Washington University's campus is "Craig's Hill," an elongated knoll in the middle of the city. This hill is almost wholly composed of the Ellensburg Formation. However, it differs in composition from the "White Bluffs" in that there is only one volcanic lahar exposed; the majority of the hill is mostly a series of thick river rock deposits known as conglomerates. 

An exposed cliff face on Craig's Hill in Ellensburg showing it's large gravel deposits. 

The Ellensburg Formation's make up should sound familiar to readers, with its interwoven gravels, sandstones, and mudflows. It is, in fact, similar to the Blakeley Formation, which lies around the shorelines of Puget Sound and similarly records early volcanic activity in the Cascades. These two geologic cousins demonstrate how volcanism has been significantly affecting the state on both sides of the cascades for millions of years, and they will continue to do so for quite some time.