Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/748

546 An argument for the great antiquity of the gravel of Milford Hill, at Salisbury, with its imbedded implements, has been justly based on the circumstance that since it was deposited the valley of the Avon has been deepened some 80 feet, and that after by far the greater part of this deepening had been accomplished, the Mammoth, Rhinoceros, Lion, Hyæna, Marmot, Lemming, &c. left their remains in the Fisherton beds. The same line of reasoning shows the far greater antiquity of the gravel on the higher plains of the New Forest, of that on both sides of the Itchen, near Southampton, and near Newport, in the Isle of Wight, more than 250 feet above neighbouring valley-gravels containing Elephas primigenius, &c.

(c) But if the great height attained by the gravel-covered plains is evidence of an antiquity far greater than that of the Milford Hill, and other high-level valley-gravels, the uniform surface of the tableland points to a continuance of similar conditions from the time of deposit of the highest gravels down to that of the gravel containing flint implements at Bournemouth, Lymington, Hill Head, &c. What these conditions were is open to question; there are no organic remains by which to decide whether the gravel covering these wide plains is an extension of the gravels of the neighbouring rivers, or a marine deposit, like that covering the southern part of Sussex.

It has already been pointed out that, probably down to the time of the low-level valley-gravels, the Isle of Wight was connected with the mainland, and a river, comprising in its drainage-area the basins of the rivers now reaching the sea at Poole, at Christchurch, and by Southampton Water, flowed by Spithead to the sea. Fig. 11 is a map of the catchment basins of the rivers in question. The area drained by all the rivers entering the sea from Poole to Portsmouth, together with the Isle of Wight, and Christchurch and Poole Bays, out to the 10-fathom line, is 2750 square miles, equal to $3⁄4$ the area of the basin of the Thames above Hampton. Of this area, 666 square miles, or nearly $1⁄4$, is comprised in the basin of the Avon, 544 square miles in that of the Test, and 479 in that of the Stour; while only 331 square miles, or less than $1⁄8$, is drained by the rivers entering Poole Harbour. The hydrographical area of the old river appears therefore to have been comparatively small, and but little augmented by the western river-basins. Within it, however, lie the large gravel-covered surfaces which have been described. Of these the New- Forest tableland alone slopes in the direction of the principal lines of drainage; it occupies the watershed between the Avon and the Test, and falls in the direction of their flow at the rate of from 18 to 34 feet per mile. The tableland on the east of Southampton Water slopes at the rate of from 30 to 120 feet per mile towards the water, and the Isle of Wight tableland at from 30 to 40 feet per mile northward. The flattest of these inclinations, that of the New-Forest tableland, is, for the course of a river, very great, and such as only