Geology Reference
In-Depth Information
Geographic distribution of individual flows and members of the Columbia River Basalt Group (after
Reidel and Tolan, 1989)
the south, widespread flows of the Prineville and
well south of its present course. Individual flows
Picture Gorge basalts in the Deschutes and John Day
periodically swept into the canyon to plug and disrupt
valleys correlate with the Grande Ronde Basalt, but
the drainage. As the frequent eruptions subsided, late
differences in chemical composition of the three lavas
Miocene and Pliocene compressional folding formed
distinguish them and indicate separate source magmas.
large-scale, east-west wrinkles that confined the river to
Prior to the extrusion of the Columbia River
its present course between Umatilla and The Dalles.
lavas, the channel of the ancestral Columbia River lay
Block diagram across the Columbia River just east
of The Dalles
