Across the Columbia Crest from Prosser (Part II)

Besides the views of the Yakima Valley, possible exploration of the old landslides and a great sense of the geometry of the Horse Heaven Hills anticline there is a great exposure of the underlying formations along the crest of the fold along the highway just below (north side) the crest of the hill.

Geology exposure along Highway 221 south of Prosser, Washington.

The units are clay, diatomite, and sandstone with a minor offset.

Elephant Mountain member basalt capping a member of the Ellensburg Formation

The Ellensburg Formation are the non basalt basalt units in between the various Columbia River Basalt Group lava flows. In this case the exposed outcrop is capped by the Elephant Mountain member of the Saddle Mountains Basalts - one of the youngest lava flows of the Columbia River Basalt Group. The member designations of the Ellensburg Formation are derived from the age of the basalt flows above and below them. On this basis the unit would be the Rattlesnake Ridge member. Laval (1956) mapped this area and proposed Prosser member for a unit containing diatomite. 

A number of distinct units can be seen within the Ellensburg Formation at this site: 1) a green-gray clay that likely highly altered volcanic ash, 2) bright white layers of diatomite, 3) cross bedded sands and silts with a mix of diatomite and volcanic ash and 4) a volcanic ash or tuff deposit.

Diatomite layers over clay and under sands. Diatomite is formed by the accumulation of diatoms, a single-celled organism with a silica structure. Diatomite signifies a quiet lake environment will little sediment input as it takes a long time for the diatoms to accumulate.

Volcanic ash overlaid by basalt of the Elephant Mountain member