Named for summit on cliffs forming north wall of Madison Canyon 5 1/2 km west of Madison Junction, Yellowstone National Park, WY, Yellowstone province. Type area designated at Mount Jackson. Includes rhyolitic lava flows that are conformable beneath Lava Creek Tuff (new) of Yellowstone Group (raised in stratigraphic rank from formation). Base not exposed at type. Is as much as 450 m thick in Madison Canyon. Exposed in vicinity of Bear Lake, south of Madison Canyon, near Wapiti Lake, west of Mirror Plateau on Flat Mountain southwest of Yellowstone Lake, and on Stonetop Mountain in the Park. The flows are varied, but all contain 30-50 percent large phenocrysts of quartz, sanidine, plagioclase, and minor opaque oxides and clinopyroxene. Is basal part of the third volcanic cycle of Yellowstone Plateau. Two flows in Madison Canyon area dated as about 790,000 and 640,000 years old. Remanent polarities of the two flows are reverse and normal. Stratigraphic table; cross section.

Source: GNU records (USGS DDS-6; Denver GNULEX).