Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/193

 the clay escarpment ; but their relation to the principal gravel-bed has not been clearly shown.

Passing from the gravels to the deposits next in age, we come to various beds of silt and silty clay underlying the most recent mud- deposits of the harbour. These beds have been exposed in clear vertical sections many hundred feet in length. In all of these sections the underlying Tertiary deposits are shown to have been cut away to a smooth, if not always to a level surface, the depth to which they have been eroded varying from a few feet above low- water level, as along part of the general Section (fig. 1), to that of from 20 to 30 feet beneath low water in the north-west portion of the area.

The surface of the mud over the whole area stands at from 6 to 7 feet above low-water level of the ordinary spring tide.

The following section (fig. 4) exhibits the principal features seen in these deposits.

Fig. 4. — Ideal Section showing the Relative Positions of the Gravel- and Mud-deposits.

x. Gravel.

A. Old mud-deposit, with stumps and roots of trees.

B. Recent mud-deposits.

C. Shingle.

In this section the gravel-bed, marked x, and the older and newer mud-deposits, A and B, are shown at their relative levels in relation to the present high and low water. It is not improbable that this section, which represents only a very small portion of the great mud-flat between Portsmouth and the foot of the chalk escarpment of Portsdown, may serve to illustrate the general condition of the surface-deposits of the harbour.

There can be little doubt that the gravel-bed (x) was at one time continuous over a great portion, if not the whole, of the surface of the harbour now covered by mud or water. At what time or in what manner it was denuded I shall not stay to consider ; it is sufficient to know that the denuding agent, whatever it may have been, has cut down to the underlying sands and clays of the Eocene- beds, which present, in all the sections I have examined, a cleanly swept surface beneath the mud.

The older and newer mud-deposits, A and B, shown in the mud-section (fig. 4), were probably formed under very similar conditions. The bed A is first seen in the sections at about 300 feet from the low gravel-capped escarpment, and spreads out northward and westward until cut off, as it were, by the deep water along the

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