. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. SECT. 2] TOPOGKAPHY OF THE DEEP-SEA FLOOR 237 A. Continental Margins a. Physiographic provinces The continental margin inchides those provinces associated vv^ith the transi- tion from continent to ocean floor. The continental margin in the Atlantic and Indian Oceans is generally composed of continental shelf, continental slope, and continental rise. A typical profile off northeastern United States is shown in Fig. 5. Gradients on the continental shelf average 1 : 1000, while on the continental slope, gradients range from 1
. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. SECT. 2] TOPOGKAPHY OF THE DEEP-SEA FLOOR 237 A. Continental Margins a. Physiographic provinces The continental margin inchides those provinces associated vv^ith the transi- tion from continent to ocean floor. The continental margin in the Atlantic and Indian Oceans is generally composed of continental shelf, continental slope, and continental rise. A typical profile off northeastern United States is shown in Fig. 5. Gradients on the continental shelf average 1 : 1000, while on the continental slope, gradients range from 1 : 40 to 1:6, and occasionally local slopes approach the vertical. The continental rise (or continental apron) lies at the base of the continental slope. Continental rise gradients average 1 : 300, but individual slope segments may be as low as 1 : 700, or as steep as 1 : 50. âI- n m -1000 â -2000fm '^3000 -4000 Sahara. â New York -Bay of Biscay - Blake Plateau Puerto Rico Fig. 6. Three categories of continental margin provinces. Category I provinces lie on the continental block, Category II provinces form the side of the continental block, and Category III provinces are the vipturned or depressed margins of the oceanic depression. (After Heezen et al., 1959.) The continental slopes are cut by many submarine canyons. Some of the larger canyons, such as the Hudson, Monterey and Congo, extend across the con- tinental rise (Fig. 7). Submarine alluvial fans extend out from the seaward ends of the larger canyons. The continental shelf and continental slope form the upper and lateral surfaces of the continental terrace (Dietz and Menard, 1951). The continental margin can be divided into three categories of provinces (Fig. 6). Category I includes the continental shelf, marginal plateaus and shallow epicontinental seas, all slightly submerged portions of the continental block. Category II includes the continental slope, marginal escarpments and the landward slopes of marginal trench
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