Figure 11.—Two east-west profiles of Southeast Newfoundland Ridge
Positions of profiles are indexed on Plate 23. Both profiles plotted from nonprecision soundings (NMC).
Southeast Newfoundland Ridge.—From the southern tip of the Grand Banks a broad ridge runs southeasterly toward the Mid-Atlantic Ridge and forms a natural barrier between the Newfoundland Basin and the North America Basin [to the south]. Since it is almost impossible to define a boundary between the continental rise and the ridge, we consider the Southeast Newfoundland Ridge an extension of the continental rise. The ridge is 60-100 miles wide, and its crest plunges southeastward from depths of 1500 fathoms near 50° W. to depths of 2200 fathoms near 45° W. Profiles N-1 and N-2 (Fig. 11) cross the Southeast Newfoundland Ridge at about 41.5° N. and 39.5° N. respectively. Profile N-1 is of poor quality, which probably accounts for the lack of fine-textured relief. The Mid-Ocean Canyon is again seen at the eastern end of Profile N-2. Profile W-5 (Pl. 24) crosses the Southeast Newfoundland Ridge from north to south. The similarity of profiles W-5 and W-23 suggests that the Southeast Newfoundland Ridge is an outer ridge of the same kind as the one east of the Blake-Bahama region. The northern one is not so long, and it does not totally enclose a basin. Otherwise, it is quite similar to the outer ridge east of the Bahamas in relative position, size, and surface features. The term Southeast Newfoundland Ridge was proposed by Wüst (1940b; 1943) and the feature has been shown on bathymetric charts (Tolstoy, 1951) and profiles (Emery, 1950). This ridge will be discussed again in connection with the Mid-Ocean Canyon and the ocean-basin floor.
Southern Grand Banks Sector.—Profiles W-4 and W-5 cross the southern tip of the Grand Banks. The shelf break is at 50 fathoms on both profiles. On Profile W-4 an apparent gradient of 1:25 extends from 200 to about 1000 fathoms where, after some irregularities probably associated with submarine canyons, the gradient drops to 1:40. This lower gradient extends to 1750 fathoms. Profile W-5 is quite similar to W-4 except that a steep initial slope of 1:5, from about 200 fathoms to 650 fathoms, is followed by a gradient of 1:80 which continues to 1000 fathoms. This same terracelike feature is also seen on W-3, W-4, W-5, W-6, W-7, and W-8. Below 1000 fathoms a gradient of about 1:50 is found on profiles W-4, W-5, W-6, W-7, and W-8. Profile W-6 runs south of the Grand Banks through the epicenter of the 1929 Grand Banks earthquake and then south through the area passed over by the 1929 Grand Banks turbidity current (Heezen and Ewing, 1952). The depression marked by the 1150-fathom sounding on the continental slope in Profile W-6 is a canyon running south from the Laurentian Channel. The continental rise is 250 miles wide and has an average gradient of 1:400 over its deepest third. At a depth of 2750 fathoms the gradient abruptly drops to 1:2000, and this marks the northern edge of the Sohm Abyssal Plain.
Figure 12.—Laurentian Channel
Profile replotted from NMC echogram
Figure 13.—Eastern Channel, Gulf of Maine