Structure of the Lower Blue Glacier, Washington

Abstract

Structural features studied in lower Blue Glacier are the foliation pattern, an unusual longitudinal septum, ogives, crevasses, and related ice fabrics. A 300-meter icefall separating the major accumulation basins from the ice tongue plays a dominant role in the formation of the principal foliated structures. Three types of ice are involved; coarse-bubbly ice, coarse-clear ice, and fine ice. Most and perhaps all the fine ice represents partly recrystallized insets or infolds of firn. An angular unconformity between stratified firn and well-foliated glacier ice is attributed to a period of high firn limits prior to 1948 and lower firn limits since. Below the firn edge the glacier displays a foliation pattern of two sets of nested arcs, convex down-glacier. These are separated by a narrow zone of strongly foliated, structurally complex ice termed the longitudinal septum. The foliation pattern becomes more irregular toward the terminus because of intersecting folia and discontinuities in strike. The ogives of the Blue Glacier appear on the surface as alternating dark and white bands conformable with the arc-shaped foliation. The dark bands are underlain by well-foliated, heterogeneous material featuring unusually large proportions of fine and coarse-clear ice. The white bands are underlain by relatively massive, uniform coarse-bubbly ice. It is inferred that the transverse foliation pattern originates in a zone of strong compressive flow immediately below the icefall. Transverse inhomogeneities created within the fall may be an important initial factor. Once formed, the foliation passively undergoes deformation within the ice tongue as it flows down the valley. The arc-shaped pattern develops within a short distance below the fall owing to differential flow. A calculation based on borehole data shows that deformation within the glacier during flow is of the correct magnitude to account for the dip of the foliation as observed at the apexes of the nested arcs, assuming that the initial attitude at the base of the icefall is essentially vertical. Complications appearing in the foliation pattern in the lower reaches of the glacier are attributed chiefly to topographic irregularities on the glacier floor, near and below the base of the icefall. It is postulated that the longitudinal septum is formed at the base of the icefall where two ice streams, split by a large rock bastion, reunite. Differences in direction and velocity of flow at the junction result in strong compression and shear which produce the intense foliation of the septum. The .high content of fine ice is attributed to the insetting and infolding of firn within the icefall and in a fosse at its base. The ogives are inferred to be primarily features formed within the icefall but subsequently modified in the zone of compressive flow at its base. The ogive dark bands may represent greatly compressed and partly recrystallized ice breccias which accumulated within crevasses formed at the lip of the icefall. There is no compelling evidence that the Blue Glacier ogives are annual features.

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