386 TRANSACTIONS OF SECTION C. 



which are very similar and both closed by harder rocks on the west — was occu- 

 pied by an eastward-flowing river, the Thames and the Solent. The valley of 

 the Thames seems merely to have deepened, retaining all along approximately 

 its original course. The valley of the Solent, on the other hand, ran for some 

 distance parallel to the sea-coast, and at no great distance from it : the result 

 of which was that the sea finally broke through the narrow ridge of chalk 

 which once ran continuously from the Needles to the Dorset coast, thus divert- 

 ing the Frome and all the western rivers from their course to the Solent and 

 isolating the Isle of Wight. This flank attack had still other effects. The 

 Lower Avon, instead of having a fall of many miles before reaching the sea 

 somewhere near Portsmouth, was shortened and reached the sea by a steep 

 direct course. Consequently, as the river flowed over loose Tertiary strata, it 

 lowered its bed so rapidly as. to cut back its valley and capture the whole 

 drainage of Salisbury Plain, which previously had followed its natural course 

 south-eastward and flowed into Southampton Water. 



To some these ideas may seem highly speculative ; but I had the good fortune 

 to discover clear evidence of this diversion. Among other things I found that 

 the high-level river-gravels of the Vale of Wardour, containing very peculiar 

 fossiliferous Purbeck-cherts derived from an outcrop at 521 feet, went straight 

 across the present Avon Valley and were found on its east side at a height of 

 380 feet ; thus showing that when the rivers which meet at Salisbury flowed at 

 a level some 300 feet higher than now they were tributaries of Southampton 

 Water. 



Thus by the diversion of these streams the great river Solent had its head 

 waters cut off and was divided into several separate river-basins, each with 

 its own outlet. This happened, I believe, in late Pliocene times. 



These flank attacks are still going on further west, and if they continue 

 much longer the breach at Lulworth Cove may widen and deepen in the same 

 way ; so that with slight submergence the so-called Isle of Purbeck may become 

 a true island, exactly comparable in its geological structure and mode of origin 

 with the Isle of Wight. 



Though at this early date the Isle of Wight was cut off from the mainland, 

 it was probably at first only cut off by a small stream and marshes, and was 

 sometimes an island, sometimes part of the mainland, as the sea-level varied. 



The final isolation took place at quite a recent period, for if Professor Ridge- 

 way and I are right, the Isle of Wight is the Ictis and Vectis of classical 

 writers, to which the ancients traded for tin. The island of Ictis is described 

 as being cut off at high tide, but connected at low tide by a narrow stone cause- 

 way. This causeway, I believe, was the ridge of Bembridge Limestone which 

 swept across what is now the Solent from Yarmouth to Hurst Castle, and was 

 intact about two thousand years ago. It has now been destroyed by the attacks 

 of the sea, and was apparently impassable even during the Roman occupation, 

 for the Roman roads seem to have led to a ferry further east and out of the 

 run of the sea. 



Thus geology and history join hands, and geology helps us to understand the 

 origin of one of the most important harbours of the world. The Solent, Spit- 

 head, and Southampton Water are parts of an ancient submerged valley-system 

 The magnificent waterways thus formed are now slowly silting up, but that the 

 process is not more rapid is due to the happy accident which diverted so much 

 of the drainage of the ancient Solent to the open sea. Had it been otherwise, 

 instead of the present fine harbours round Portsmouth we should have had a 

 series of alluvial flats and sand banks such as now block the lower reaches of 

 the Thames. 



2. Constructive Waterfalls. By Professor J. W. Gregory, F.R.S. 

 See Section E, p. 445. 



3. Tidal Movements of the Deep Water of the Skagerrak, and (heir Influence 

 upon the Herring Fishery. By Professor 0. Pettersson. — See 

 Section E, p. 446. 



