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 volc