Grinnell Glacier, Glacier National Park, Montana

Stratification due to the annual accumulation of snow is well exhibited in Grinnell Glacier. Layers of snow in the narrow firn zone at the head of the glacier are wedge-shaped and have a steep initial tilt toward the glacier front. Weight of overlying layers forces the earlier-formed layers to rotate so that after travelling a relatively short distance they assume a backward dip which gradually increases until, approximately halfway between cirque wall and glacier front, a maximum of 45 degrees is attained. Detailed study of the position of ice strata in various parts of the glacier has yielded information on the nature of the ice motion, the age of the ice, and its probable thickness. Available records dating back to 1897 and direct observation by the authors over a 4-year period indicate that Grinnell Glacier has been reduced to about half the size it was in 1900, and that the recession during recent years has been most rapid.