Since the sea covers the greater part of the earth's surface it is quite reasonable to regard the sea-floor as the basic form of the crust of the earth, with superimposed upon it the continents, together with the islands and other features of the oceans. (1)_______________________. From the shore-line out to a distance which may be anywhere from a few miles to a few hundred miles runs the gentle slope of the continental shelf, geologically part of the continents. (2)_______________________. This continental slope usually starts at a place somewhere near the ice-fathom mark and in the course of a few hundred miles reaches the true ocean-floor at 2,500-3,000 fathoms. The slope averages about 1 in 30, but contains steep, probably vertical, cliffs, and gentle sediment-covered terraces, and near its lower reaches there is a long tailing-off which is almost certainly the result of material transported out to deep water after being eroded from the continental masses.
发表于 2012-1-12 10:32
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发表于 2012-1-12 15:02
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