But every now and then it is worth taking a few extra minutes to get off the interstate for a short side trip. Interstate 90 is the main east-west route across the state. One short side trip along this route that won't delay the east-west traverse very much is to take State Highway 10 between Ellensburg and Cle Ellum. It won't add any miles to the trip but has some spectacular outcrop exposures of the Ellensburg Formation. There are two broad pullouts and the road is wide enough that it is a great spot to take a break and contemplate the violence of the Cascade volcanics during Miocene.
Locations of Ellensburg Formation exposures along Highway 10 between Cle Ellum and Ellensburg (Google Maps)
Volcanic ash, lahar and volcanic sediment
Graded lahar and ash
Ash and lava fragments
The three pictures above are from the western stop. There are multiple deposits and interesting cross-cutting features.
Lahars and volcanic sediment
Pink conglomerate boulder of older lahar embedded within the lahar
Cobble within graded lahar deposit
Conglomerate at base of lahar deposit
The Ellensburg Formation is the name given to units of alluvial and volcanic material between basalt flows of the Columbia River Basalts. The basalts primarily originated as massive huge flows from eruptive centers near the Oregon, Washington and Idaho border area. Some of these flows extended all the way to the Pacific Ocean. The Cascade Range was lower at the time, but there were explosive volcanoes in the south part of the range. The bulk of material from the Ellensburg Formation was derived from those ancient eruptive centers that are long gone and now replaced by the current crop of Cascade volcanoes. The Ellensburg Formation can be thought of as the inter fingering of volcanics and alluvial material from the west with basalts from the east with the basalt being the predominant formation.
The second stop has nice views of the adjoining Yakima River that carved down through the Ellensburg Formation providing the views of the unit. An old timber and metal irrigation aqueduct that is no longer functional traverses the slope both below and above the road at the outcrop location.