The old sea-bed of the Sussex level, then, appears to pass between the old coast-line thus marked out on the north, and the beach-deposits of the Foreland, St. Helens, &c. along the coast of the Isle of Wight, becoming more like a river-gravel in character, and containing numerousflint implements in its westward extension. The supposition that there was here an estuary through which the rivers from the westward reached the sea when the Isle of Wight was still joined to the mainland, derives support from other considerations.
(a) The connexion of the island with the mainland, which is obviously suggested by the continuation of the chalk range of Purbeck through the island, appears to have existed down to recent times. The ten-fathom line, which is within half a mile of the coast all along the back of the Isle of Wight, strikes across Christchurch and Poole Bays to Handfast Point, following the course of the vertical chalk strata, and again hugs the shore round the Purbeck coast. Inside this line the sea is generally less than five fathoms deep, and is steadily encroaching on the land at the rate of about a yard in a year. At this rate the coast would have receded from the line of the chalk to its present position in about 9000 years. This is of course but a rough estimate of time, but it shows the probability of a connexion by which the Mammoth and tichorhine Rhinoceros had free access to the Isle of Wight down to the time of the lower valley-gravels, in which their remains are far from rare.
The Chines, or bunnies, along the coast between Hordwell and Poole appear to show that a change has taken place in the course by which the streams reached the sea, as has been already pointed out. The extensive mud-flats more than a mile wide along the north shore of the Solent must, as Mr. Godwin-Austen has observed[1], have originated in a very different condition of the Solent from the present, and they also point to the comparatively recent time at which the condition was that of an estuary, in which the formation of such mud-flats would have been natural.
When a continuous range of chalk downs stretched along where the ten-fathom line now lies between the Needles and Handfast Point, the river system was most probably analogous to that which now exists. All the streams which traverse the chalk of the Isle of Wight and Purbeck, flow northwards from the older beds into the Tertiary basin; and it appears unlikely that the rivers which now enter the sea at Poole and Christchurch took an opposite course through the chalk. It is natural to suppose therefore that the Frome and Piddle continued their easterly course inside the chalk range in a joint stream, of which the Avon and Stour were affluents, and which received tributaries draining country up to, and perhaps beyond, the chalk on the south, as the Corfe River, the Medina, and the Brading river still do. The rivers flowing into Southampton Water joined those coming from the westward by way of the Solent, and formed a broad estuary communicating with the sea by Spithead.
- ↑ Quart. Journ. Geol. Soc. vol. xiii. p. 65.
