SINCE 1931 the National Park Service has measured the yearly recession of the fronts of several glaciers in Glacier National Park, Montana. Such measurements, however, usually made at only one or two points along the front, give little indication of the amount of ice shrinkage. Consequently, in 1937 the Park Service initiated a mapping program with the preparation, on the scale of : 2400, of a detailed topographic map of Grinnell Glacier. Similar maps were made of Sperry Glacier in 1938, Jackson Glacier in 1939, and Sperry and Grinnell Glaciers in 1946.' The remapping of Sperry and Grinnell has yielded comprehensive quantitative determinations of their shrinkage between the surveys. They show a phenomenal reduction in volume, expressed both in lowering of the surface and in decrease in area.