What is interesting about this picture is it represents the horizon between two separate flows. The upper layer represents the Frenchman Springs Basalt, the lower member of the Wanapum Flow. The Wanapum flow originated from dikes between Pendleton and Kennewick, WA about 15.3Ma and covered.
In between the two basalt flows, there is a baked layer of paleosoils 0.5-1.0 m thick. This red layer is the Vantage Soil Horizon and represents soils weathered in 200,000 years between flood basalts.
The older (lower) Grande Ronde Flow originated NE Oregon ending about 15.5 Ma. The Grande Ronde flow was the most extensive of the CRBs with as many as 18 individual flows covering up to 160,000 km2 with at basalts. Sentinel Bluffs layers are generally only 10-30m thick, but some of the lower Grande Ronde flows were over 80m thick.
|Figure Taken From http://geology.isu.edu/Digital_Geology_Idaho/Module10/mod10.htm|
This particular member of the Grande Ronde Flow originated from the Chief Joseph Dike Swarm, a series of dikes about 100 miles NE of this outcrop. This is interesting in the fact that today, these dikes are separated from this outcrop by the broad anticline which makes the core of the Blue Mountains
Fens, McConnelll, and Ely, 2004, "Field Trip Guide to the Geology of the Umatilla River Basin October 14 and 15, 2004" Oregon Department of Geology and Mineral Industries Open File Report OFR-O-04-23.