As agents of erosion, ocean-currents are not, in general, of great importance. Currents which reach the bottom are comparable, in their effects, to rivers of the same velocity and volume; but most ocean-currents do not touch bottom, and, therefore, do not erode it. Where the current agitates the bottom sensibly, as it often does in shallow water, the bottom is abraded, and in the lee of such places it is doubtless aggraded. Since ocean-currents do not, for the most part, flow in shallow water, their erosive work is, on the whole, relatively slight; but where they are forced through narrow and shallow passageways, their abrasive work may be considerable. Thus the Gulf Stream, where it issues from the Gulf, has a velocity of four or five miles per hour, and its shallow and narrow channel is current-swept. A rough test of the abrasive work of an ocean-current is found in the nature of the bottom beneath it. If this be hard, it indicates that the loose sediment on the floor of the ocean has been swept away, while the presence of fine detritus indicates that the current is not wearing. Thus the abrasive power of the Gulf Stream is known to continue somewhat beyond its narrow channel, for on the Blake plateau (between the Bahamas and Cape Hatteras), where the water is 600 fathoms and less in depth, “the bottom of the Gulf Stream ... is swept clean of lime and ooze and is nearly barren of animal life.”[168] Other illustrations of the erosive power of currents have been noted near Gibraltar in water 500 fathoms deep, and between the Canary Islands at depths of 1000 fathoms.[169] In spite of these examples, and of many others which probably exist in similar situations, it yet remains true that ocean-currents are on the whole but feeble agents of erosion.