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deepwater plains of ocean basins and depressions of epicontinental seas. They are found within the bounds of the ocean floor and in the transitional zone and are divided into two morphological types—flat, or subhorizontal, and hilly. Flat abyssal plains are confined to the margins of ocean basins or to the centers of the basins of epicontinental seas. Hilly abyssal plains occupy a large part of the bottom of the ocean floor. The surface of the flat abyssal plain is characterized by exceptional evenness, with floor gradients of less than 0.001. Starkly contrasting solitary seamounts, evidently of volcanic origin, are also found. Flat abyssal plains are most widely found in the Atlantic (Hatteras, Biscay, Angola, and so forth). Their evenness is caused by the fact that they are unique traps that catch loose sediment (foraminiferal and diatomaceous silts and marine glacial deposits in polar waters).
The hilly abyssal plains show significant ruggedness. Relative variations of depth are from 500 to 1,000 m. The surface is a combination of hills and ridges separated by depressions; solitary seamounts are also frequently found. The floor is covered by deepwater deposits. Hilly plains are most widely found in the Pacific and Indian oceans and to a lesser degree in the Atlantic.