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Color-coded shaded-relief bathymetry of Gull Island Reef. The morphology is similar to that of Boulder Reef. Underwater video shows the rough relief is composed of cobble and boulder substrate with less than a meter of local variation (Greg Kennedy, U.S. Geological Survey, Ann Arbor, Michigan, personal commun., 2002). The arcuate shape of the shallow ridge, 3 m depth at (A), suggests derivation from a small lobe of ice moving eastward. This contrasts with the southward ice motion suggested by the morphology at Boulder Reef. Erosional and (or) depositional ridges radiate outward to the northeast from the reef, including a straight, 1- km-long ridge (B). Along the north and east edges of the reef, bottom samples suggest that a smooth, modern sand sheet drapes the rougher substrate (C).