The origin of these harbours is quite easy to understand, if we admit the recent sinking of the land, and for this we will presently give abundant evidence. On any other hypothesis these inosculating waterways must seem hopelessly confused and inexplicable. Sea and waves do not erode enclosed basins such as these. Granting the submergence, we see that each of these harbours must once have been a shallow valley; but this does not account for their basin-like shape and their cross connexions. For the reason of these peculiar features we must look at the map by the Geological Survey showing the superficial deposits. It will then be seen that all this part of Hampshire shows a widespread sheet of gravel and gravelly loam which slopes gently seaward and passes below the sea at Spithead. Northward the gravel rises, and the soft Eocene and Cretaceous strata appear beneath the gravel between tide-marks at various places toward the northern ends of these harbours. The waves of the sea can remove loose gravel as readily as clay, and we see that on this coast wave-action is practically confined to the low cliff facing the sea and does not affect the interior of the harbour. But it is well known to geologists that a sheet of coarse angular gravel such as this, notwithstanding its looseness, is much less readily attacked by a small stream than is a surface of hard clay or even chalk. Thus plains of Tertiary deposits capped by gravel, under the action of rain or rivers develop into gravel-capped plateaus and hills, which fall abruptly into open flat-bottomed valleys. The denudation takes place at the edge, where the gravel rests on the Tertiary strata and numerous springs are given out; there is scarcely any denudation in the gravel flat, and unless the height of the land is considerable there is no great amount of denudation in the flat bottom of the valley. Thus there is a tendency for the valley to widen out on every side, wherever the gravel rests on impervious or soft strata. But where the gravel plunges below the water-level, as it did at the entrance to each of these harbours, the valley narrowed, for there were no landslips, the drainage was subterranean, and the stream could not readily remove the large flints. The widening of the valleys, where they were cut in soft strata, led to the development of small lateral valleys to the right and left, leaving only narrow divides between their head waters and those of the next valley. When the land sank these divides were flooded, and so were developed the shallow cross connexions, much as we now see them.