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Long reversed refraction profiles along the outer edge of the continental terrace have been made in numerous locations from Guatemala to the Bering Sea to determine the nature of the crustal section beneath the sediments. Off Guatemala, the structure is consistent with the theory that volcanic rocks and sediments have been laid down over a typical oceanic section, depressing the Moho as the upper surface built up to Pleistocene sea-level. Off western Alaska a thick section of material with granitic velocity is overlain by thick sediments; the Moho is down almost to continental depth, and the section appears to be similar to the adjacent continent.
Off the coasts of Canada, California, and Mexico, a more confusing structure is found. Depth to the Moho is between the Alaskan and Guatemalan values but does not show a progressive change. At some stations, the crustal velocities are similar to the continent; at other stations, crustal velocities are oceanic. In some places, the mantle velocity is abnormally low.
The depth to the Moho at all stations from Alaska to Guatemala is intermediate between continental and oceanic values. The mean depth is 21 km. From the data now on hand, one can only conclude that the crustal structure beneath the shelf is not uniform and cannot be reconciled with any simple hypothesis of either stability or growth of the continental mass.
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