Fig. 1.

Map of ice streams, ice rises, and grounding zones of West Antarctica and their associated flow bands on the Ross Ice Shelf. The heavy shadings are the marginal shear zones, and light shadings are the relatively less chaotic parts of the ice streams. The base camps (UP, DB, UC, DC) GSIRE stations (dots), RIGGS stations (open circles), and IGV traverse stations (open squares) are shown. The ice rises are shown with solid outlines and no shading (e.g. “a”, “A”, C.I.R., Steershead). There are four narrow strips (no shading) around C.I.R. (Crary Ice Rise) that we have interpreted also as ice rises. The parallel shading around ice rises denotes heavily crevassed marginal areas. Grounding lines are shown as a band of cross-hatching (the grounding lines for ice streams D and E are not shown). The parallel hatching around and upstream of C.I.R. is a zone of intensive crevassing and is partially grounded in some locations. The areas marked with a wavy pattern are zones of surface and bottom rumples, wherein we believe the ice shelf to be partially grounded. Contours (solid lines) are at 10 m intervals up to 120 m, at 20 m intervals from 120-200 m, and at 40 m intervals at higher elevations.

Elevations above sea level were determined from Doppler-satellite heights above the reference ellipsoid and the GEM 10C geoidal model. A regression line of height above sea level vs. ice thickness proved useful for distinguishing between floating and grounded ice. Deviations from hydrostatic equilibrium of about 10 m or more were detectable. Application of this criterion has led to detection of several ice rises and ice rumples (Figure 1).

The surfaces of the active ice streams, A and B, are heavily crevassed and their boundaries are marked by chaotic, incoherent bands of surface crevasses, but there are no visible crevasses on ice stream C. However, sounding on the surface of ice stream C, using a carrier-free monopulse radar, has revealed numerous crevasses buried at a depth of about 35 m, which corresponds to about 250 years of snow accumulation. This suggests that ice stream C ceased to be active about 250 years ago.

Crevasses at or near the surface produce a distinctive back-scattered radar return (clutter) that we have used to map the ice streams and their extensions onto the ice shelf. The lack of “clutter” on slowly-moving ice, along with the surface elevation data, has enabled us to extend the mapped boundary of Crary Ice Rise about 70 km upstream, and to map several other ice rises (Figure 1).