During the Holocene epoch, volcanic activity has been particularly concentrated between the North Sister volcano and Three Fingered Jack, with at least 125 distinct eruptive centers active since the retreat of Pleistocene glaciers. Eruptive activity in the upper McKenzie River catchment has been taking place since after the eruption of Mount Mazama about 7,700 years ago. Thus, it represents one of the youngest volcanic regions in the Cascade Arc. This activity is concentrated in the central Oregon Cascades, at the northwestern margin of the High Cascades graben near the boundary of the High Cascades and Western Cascades.