Even Southampton Water and the Solent themselves are nothing but submerged valleys. A well at the Horse Sand Fort—one of the iron forts which rises out of the sea at Spithead—showed a band of compressed vegetable matter, probably an old land-surface, more than 50 feet below high-water level, the floor of Eocene strata not being met with till 98 feet below high water was reached. In this case, however, the strata below 50 feet seem, from the published description, to be of marine origin.
The well at Norman Fort is stated to have penetrated to a depth of 127 feet below the sea before Eocene strata were reached; but in this case the lower strata were of marine origin, and the only land animal recorded was a jaw of red deer, found apparently between 80 and 90 feet down. These deep channels may be relics of the very ancient (Tertiary) Solent River, and were probably arms of the sea till they were sufficiently silted up for the lowest submerged forest to grow.
We have not yet sufficient data, nor is it necessary to our purpose, to give a detailed reconstruction of this interesting area during the successive stages of elevation and depression. During the time when the lowest of the submerged forests flourished the Isle of Wight was connected with the mainland where the Solent now narrows about Yarmouth, and probably for some distance westward. This connexion was kept up till comparatively recent times, only breaking down finally a short time before Caesar’s invasion.
In early Neolithic times the ancient Solent Valley had already been decapitated by the inroads of the sea west of the Needles, and the remains of this big valley were occupied by a small river flowing eastward through the middle of the present Solent. In its course it received numerous tributaries on either side. It probably opened out into an estuary where it joined Southampton Water, and so continued to and beyond Spithead, receiving other tributaries from the valleys now occupied by Portsmouth Harbour, Langston Harbour and Chichester Harbour.
In time we may be able to make a more complete reconstruction of the physical geography of this area for definite dates; but the point now to be insisted on is that the Isle of Wight was part of the mainland up to quite recent times, so that its fauna and flora could readily pass backwards and forwards without crossing the sea.
Perhaps to the geographer or geologist one of the most striking confirmations of a recent submergence affecting this part of England will be found in the strange series of enclosed harbours extending from Chichester westward to Fareham. These harbours are not each distinct and separate; all of them have cross connexions in the form of shallower channels some four or five miles inland from Spithead. I have often been asked what is the meaning and origin of these peculiar harbours, which are not forming or widening now, but rather tend to silt up.
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.