The basalt flows that erupted have been divided into groups based on the timing and location of the eruptions, in addition to their geochemical signatures. Both trace element data and radiogenic isotopes have been used to group the basalts.
This figure shows the temporal and spatial evolution of some of the basalt flows of the Columbia River Group. (Figure from Camp, 1995).
The oldest group of the Columbia River Flood Basalts (not shown in the above figure) is the Picture Gorge Basalt.
The main group, which is slightly younger is the Clarkston Group. It comprises 97% of the total lava erupted by volume. The main member of this group is the Grande Ronde Basalt formation, which comprises 85% of the total volume of basalt erupted (Hooper and Hawkesworth, 1993). Basalts within the same group are presumed to come from a similar source.
The youngest group is the Saddle Mountain Basalt. It is much smaller in volume and erupted over a longer period of time.
To learn about the initial controversy over the CRF basalts click on old controversy