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VII] extremely hard, and can resist for ages the attacks of the sea; but loose Tertiary material, or even Triassic strata, would readily be swept away by the heavy Atlantic swell and by the scour of the tides, until they were protected by the building of Plymouth Breakwater.

There is a general impression that marine action cannot go on much below low water; but this is altogether a mistake. Tidal scour may go on at any depth, provided the current is confined to a narrow channel, so as to obtain the requisite velocity. If in addition there is a to-and-fro motion, such as that caused by the Atlantic swell at depths of at least 50 fathoms, the actual current required to remove even coarse sand need only be very gentle. The oscillation in one direction may not reach the critical velocity; in the other this velocity may just be exceeded; the movement, therefore, of the sand grains may always be in one direction, especially if the courses taken by the ebb and flood tides do not coincide, or their velocities differ.

How does this apply to the origin of Plymouth Sound? The mere fact that opposite the mouth of the Tamar a pit has been scoured to a depth of 150 feet, and opposite the Hamoaze another to 132 feet below low water, and that these pits are kept open, notwithstanding the enormous amount of sediment brought down by these rivers, proves that tidal scour