RIFT VALLEY: The most striking feature on an average profile across the Mid-Atlantic Ridge is a deep notch or cleft in the crest of the ridge. In a small percentage of the sounding profiles two or three such valleys are present, and on a few profiles no notable depressions are observed. On an average profile the floor of the valley lies at about 2000 fathoms, while the adjacent peaks average about 1000 fathoms below the sea surface. The width of the valley between the crests of the adjacent peaks ranges between 15 and 30 miles, and the depth of the valley floor beneath the highest adjacent peak ranges from 700 to 2100 fathoms. The width of the valley 500 fathoms above its floor ranges from 5 to 22 miles. The range in observed depths of the valley is 1150 to 2850 fathoms in the area of the physiographic diagram. The adjacent peaks range from 500 to 1300 fathoms within the same area (excluding the area near the Azores) (Fig. 47). [Illustration: FIGURE 37.--_Natural-scale Mid-Atlantic Ridge profile 1A_ Slope corrections have been applied to profiles 1 and 2. The method is described by Elmendorf and Heezen (1957).] [Illustration: FIGURE 38.--_Natural-scale Mid-Atlantic Ridge profile 1B_] [Illustration: FIGURE 39.--_Natural-scale Mid-Atlantic Ridge profile 1C_] [Illustration: FIGURE 40.--_Natural-scale Mid-Atlantic Ridge profile 2A_] [Illustration: FIGURE 41.--_Natural-scale Mid-Atlantic Ridge profile 2B_] [Illustration: FIGURE 42.--_Type profile, provinces of the Mid-Atlantic Ridge_] [Illustration: FIGURE 43.--_Tracing of PDR record, Rift Valley, Rift Mountains, High Fractured Plateau, and Upper Step_] [Illustration: FIGURE 44.--_Tracing of PDR record, Western Rift Mountains_] Twenty-six crossings of the Rift Valley are shown in Figure 45. The profiles can be divided into three groups: (1) single well-developed rift valley; (2) several well-developed deep valleys; (3) no particularly deep central valley. Most of the profiles (20) fall into the first class; the second class is represented by 5, and only 1 falls in the third class. The topography of the floor of the rift is rough. In no instance has a flat floor been observed. Where the valley is widest mountains a few hundred fathoms high protrude from its floor. 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. [Illustration: FIGURE 45.--_Twenty-six rift valley profiles, Mid-Atlantic Ridge_