These high tides of enormously distant past ages constitute the denuding agent which the geologist required.
Of all denuding agents, there can be no doubt that, to the land exposed to them, the waves of the sea are by far the most powerful.
The waves are a great planing machine attacking the land, and the tides raise and lower this planing machine, so that its denuding tooth is applied, now twenty feet vertically above mean level, now twenty feet below.