Muldrow Glacier in Alaska advanced extremely rapidly during the winter of 1956–1957, with surface movements of ice amounting to over 6.6 km. This flow was accompanied by a pronounced lowering of the surface of the upper portion of the glacier by 170 m, or more, with a corresponding rise in surface ice levels in the terminal portion; the net exchange of ice is roughly computed to be equal. No other glacier in the region showed evidence of a similar movement during the same period. However, the Black Rapids Glacier in 1937, the Yanert in 1942, and the Susitna in 1953 did make advances of a similar nature. Unusual medial moraines indicate that at least two of these glaciers have previously alternated between stagnation and sudden, brief movements. No evidence was found to suggest short-term climatic conditions favorable to large snow accumulation on these glaciers prior to their advances. Correlation between local earthquakes and these ice movements is also lacking. Evidence does indicate that these advances result from critical dynamic conditions met with in certain glacier systems favoring long periods of stagnation followed by sudden displacements of ice.