52 



Bournemouth Bay. To the north lies the Thames basin, to which 

 it is very similar. Each has a river system draining it eastward, 

 the river Thames in the northern basin, and in the Hampshire 

 basin we find the remains of what was once a great river system in 

 the river Frome at Wareham, Poole Harbour, and the Solent. 

 Tributary streams in each basin flow north and south to the main 

 river from the chalk ridges. The South Downs form the water- 

 parting between the two. Formerly these two basins were con- 

 tinuous, their contents being laid down before the upheaval of the 

 Downs divided them. In the Hampshire basin the chalk forms its 

 floor, and dips down gently from the South Downs under the Solent 

 to turn up again in the Isle of Wight, where it becomes nearly 

 vertical, under the Tertiary beds which fill up the basin. 



The plateau of chalk hills dividing the Thames basin from 

 Hampshire continues to the east of the Wealden district, where it 

 formerly formed the water-parting dividing the rivers of the Weald 

 into north and south streams, but it has since been denuded off, 

 leaving the Wealden strata exposed ; this has since been raised up 

 and now forms a plateau. 



In the Hampshire basin most of the rivers flow southward, 

 such as the Avon and Stour. There are a few relics of those which 

 formerly flowed northwards in the Yar and Medina of the Isle of 

 Wight. Southampton Water is a submerged valley, in which 

 formerly a river flowed southward to the old river Solent, as the 

 original great river of geological times has been called. This once 

 formed a river system corresponding to that of the Thames and its 

 tributaries in the northern basin. 



The Hampshire basin was formed by great earth movements, 

 which occurred in comparatively recent geological times. To 

 understand these movements we must remember that the centre of 

 the earth is rigid, and twice as solid as a globe of steel. Earth 

 waves travel round the crust. As the earth cools it contracts, and 

 the crust has to wrinkle up to adjust itself to the diminished 

 volume of the interior. The lower portion of the crust consists of 

 ancient rocks, such as granite, gneiss, etc., under intense pressure. 

 These do not split or fracture, but flow, being very slowly compelled 

 to this by the enormous pressure of rocks above them. But the 

 more superficial stratified rocks, which cover them, and are far less 

 rigid, have to slide and slip and ride over one another. The chalk 

 floor of the basin was deposited very slowly in a sea which 

 subsequently became dry land, and on which — after some erosion — 

 the Tertiary beds were laid down. At the close of this period an 

 enormous outburst of volcanic activity occurred in many parts of 

 the globe, especially in the West of Scotland and North of Ireland, 

 where the remains of vast lava fields occur, which are now over 

 3,000 feet thick in places, and formerly must have been of even 

 greater depth. It was probably the extrusion of this enormous 

 quantity of molten rock which reduced the pressure and support of 

 the earth's crust in the South of England, and so caused the sag 

 in the surface which we now recognise as the Hampshire basin. 

 Similar earth movements have occurred many times in geological 



