Seismic-refraction profiles shot on the continental rise and slope south of the Grand Banks show great sediment thickness which reaches a maximum of nearly 10 km about 90 km south of the 100-fathom curve and thins rapidly under the banks. Three sediment layers are present, with average velocities of 1.83, 2.47, and 3.97 km per second, the highest velocity layer constituting the major portion of the column. The basement is thickest under the south edge of the continental shelf, thinning to 2 or 3 km to the north and to the south. Its velocity averages 5.77 km/sec except on the southernmost profile where it is 6.70 km/sec. Sub-basement is reached on the three southernmost profiles at an average depth of about 13 km, the velocity decreasing from south to north from 7.76 km/sec to 7.24 km/sec.

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