The White River Ash is a bi-lobate 1500 year old deposit occupying at least 6 cubic miles and covering some 125 000 square miles of southern Yukon and eastern Alaska. Sixty-six samples were collected at 5-mile intervals, principally along two traverses 120 miles apart across the main lobe, and subjected to X-ray fluorescence and petrographic analysis.The ash is a rhyodacite composed of glass (n = 1.502), andesine, hornblende, hypersthene, and magnetite. The average chemical composition is SiO2 = 67.4, Al2O3 = 15.1, TiO2 = 0.5, MgO = 2.0, FeO = 2.0, Fe2O3 = 2.2, Na2O = 4.1, K2O = 2.5 and CaO = 4.1, but there is a significant difference between the two traverses owing to the increase in glass relative to crystal components downwind.A synthesis of the distribution of the ash permitted the drawing of a 5 by 12 miles 'target' source rectangle in the St. Elias Range between Mts. Natazhat and Bona in Alaska. Aerial photographs revealed a suspect mound 0.4 miles in diameter beside the Klutlan Glacier. Access by helicopter showed the mound to be a flat cone of large White River pumice fragments. It is believed that the vent lies beneath the glacier next to the cone.