RIFT MOUNTAINS: The steep walls flanking the rift each form one side of a large rough-sided block. They might be considered as tilted blocks whose facing slopes form the Rift Valley. The back or outer slope of the Rift-Mountains Province is generally broken into mountains as much as 500 fathoms high and 10 miles wide (Fig. 44). The lateral limit of the Rift-Mountains Province is reached when the average slope of the sea floor flattens markedly. Because of the high local relief it is sometimes difficult to pick the boundary of the Rift Mountains, but in almost all recorded profiles the approximate position of the boundary is unmistakable.
Figure 45.—Twenty-six rift valley profiles, Mid-Atlantic Ridge
Position of profiles shown on Plate 23
Figure 46.—Five representative profiles, crest and western flank of Mid-Atlantic Ridge
HIGH FRACTURED PLATEAU: The High Fractured Plateau is adjacent to the Rift Mountains on either side of the ridge (Fig. 42). The local relief is about 400 fathoms from peak to adjacent valley, and the distance from peak to peak ranges from 8 to 20 miles. In contrast to the adjacent flank provinces there are no filled intermontane valleys, and the valleys are deeper and narrower than in the adjacent Upper Step Province. Within the limits of the physiographic diagram, the average depth of the High Fractured Plateau ranges from 1500 to 1900 fathoms.