The continental shelf is the first province we encounter on leaving land. The average width of the shelf is approximately 40 miles, but in some places, there is no shelf (for example, along the west coast of South America). The widest shelf is found along the glaciated coast of Siberia, where it extends out roughly 800 miles. Continental shelves comprise about 7.5 percent of the total ocean bottom.

The shelf has a very gradual slope. It declines at an average rate of 2 fathoms per mile, and at

Figure 1-3-1.-The five major bottom provinces.

its seaward limit. the water above the shelf is usually 60 to 100 fathoms deep (1 fathom = 6 feet). Although the average slope of the shelf is gradual, terraces, ridges, hills, depressions, and deep canyons are found within its boundaries. The shelf region is a transition zone between freshwater runoff from land and the more saline water of the sea; consequently, it is an area of great mixing of water with generally unstable water conditions. Currents normally run parallel to the shore in this region.

At the seaward edge of the continental shelf the slope becomes much steeper. This region is known as the shelf break. The drop off is rapid. On the average, the slant ratio is roughly 20 times greater than that of the continental shelf. The ratio is generally much greater off mountainous coasts than off wide, well-drained plains. On bottom contour charts (figure 1-3-2), the bottom contours are tightly packed, thereby reflecting the much steeper gradient.

The continental slope resembles a steep cliff that has been eroded by heavy rains. Its most striking features are the submarine canyons, deep cuts or scars, that are prevalent along the slope face. These canyons are thought to have been formed (or cut out) by turbidity currents, which are dense, sediment-laden currents that flow along the ocean floor. Some of these canyons are equal in size to the Grand Canyon. At the seaward end of these canyons, large amounts of sediment are deposited and spread out in a fan-like manner to form the continental rise. Refer to figure 1-3-1 again.

The continental rise is found seaward of the continental slope, in approximately 500 fathoms of water. It is made up of thick sediment deposits that cover irregular relief features. These deposits slope gently seaward forming the abyssal plains of the deep ocean basins. At the seaward edge of

Figure 1-3-2.-Bottom contour chart.

the continental rise, the water depth is about 1,500 fathoms.

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