A recent day of field work which mostly entailed navigating complex topography to get to where I needed go had lots of basalt outcrops of the Grande Ronde basalt member of the Columbia River Basalt Group (CRBG). The Grande Ronde is the most massive of the lava flows that make up the CRBG. The two flow sections I was navigating through west of the Kittitas Valley are interpreted by Tabor and others (1982) on the Wenatchee 1:100,000 geology map as two upper flows of the Grande Ronde based on stratigraphic position and polarity of the iron minerals in the flow. These are the same flows that are exposed in the bottom off the Grande Ronde River Canyon in the southeast corner of the state grande-ronde-basalts-at-grand-ronde.
Cliffs of basalt
Top of cliff
Scree of weathered basalt
The Tabor and others (1982) map indicates Ellensburg Formation volcanics about a mile to the east. This exposure fits the Ellensburg Formation from the area. The Ellensburg Formation does interfinger with CRBG meaning that when the early massive CRBG flood basalts flowed across southern Washington approximatley 16 million years ago there was also a rather violent separate volcanic field in the area with explosive eruptions. Some of the explosive eruptions can be viewed along Highway 10 northwest of Ellensburg ellensburg-formation-side-trip