Sir,

For a decade prior to a hot, dry Summer in 1958, most glaciers in the north-western United States increased in volume.Reference Hubley ^{1 ,} Reference Bengtson ^{2 ,} Reference Harrison ³ Measurements were made during this period at the Coleman Glacier on Mt. Baker, Washington, and the resulting data indicate interesting relationships between ice flow and ice thickness. The ice flow is apparently not proportional to the thickness or gradient .but may depend on a longitudinal compressive force transmitted from more active ice upstream. The dataReference Harrison ³ show that the advance of the Coleman Glacier ended abruptly after a net average loss in thickness of 7 m. over the entire glacier in 1958. The ice thickness at the terminal tongue decreased 15 m. between June and September of that year. However, the thickness of the tongue was still greater than in previous years when the ice was advancing rapidly. Although the advance was greatest between 1954 and 1955, the volume increase at the tongue was greatest during the 1955–56 season. The flow into the tongue below an elevation of 1,500 m. was reduced after a warm Summer in 1956, probably as a result of a decrease in ice thickness in the region between 1,500 and 1,800 m.

Data on the annual advance (possible error ±2 m.) at the Coleman Glacier front since 1954 are:

Supplementary data on the ice thickness at the tongue are illustrated by the profiles plotted in Figure 1. The gradient is shown without exaggeration; it decreases abruptly at approximately 1,500 m. The change, which is marked by a series of crevasses, defines the brink of the ice fall shown in the profiles. Above this elevation is a large, relatively flat region which feeds the terminal tongue which is not shown in Figure 1. The thickness at the brink of the ice fall could be estimated because a rock cliff was exposed near this point until 1956. The advances shown in the illustration are generally indicative of the response at the front but do not check with the tabulated data because the direction of advance did not follow the line of this profile. The 1959 and 1960 profiles are nearly identical with the 1958 profile and are not plotted.

Fig. 1.

Profile of Coleman Glacier tongue at end of ablation season

Although the tongue thickness was greater in 1958 than in 1954, except at the brink of the ice fall, the flow of ice was drastically reduced after a single Summer of unusual melting. The convex, bulging front which is characteristic of active ice flow was replaced by a tapered front which appeared to be sliding or moving along shear planes. Meager flow in 1958–59 was offset by melting, but the front plowed ahead 24m. during the Winter of 1959–60, pushing up a small moraine. This advance was nearly destroyed by increased melting during the Summer.