78 



the south side of the valley, opposite Bournemouth, and for the 

 formation of Bournemouth Bay ; it took place after the first 

 glaciation, or we should find the foreign erratics near Bournemouth 

 and Poole, just as they are found at Selsey. 



I want particularly to draw attention to this recent date for 

 the inroad of the sea into the Solent valley opposite Bournemouth. 

 It is a new point. Formerly I thought that this gap dated back 

 to the Pliocene period ; but closer studies convince me that the 

 breach was only made after, not before, the time of maximum 

 glaciation. Man may well have seen this great change in physical 

 geography, and may have watched the sea break through the wall 

 of chalk during some storm. 



This will give some idea as to how a geologist reconstructs 

 ancient lands, and how he correlates changes, of land and sea with 

 particular periods of heat or cold, the date of which is already 

 known approximately. We may take it that during the first 

 glaciation Bournemouth lay on the shore of an Arctic river, and 

 was far distant from the sea. Look at Lyell's picture* of the 

 shores of the River St. Lawrence, strewn with far-transported 

 erratics, brought by the floating ice in spring. This gives a fair 

 idea of what the Solent River was like during the first glaciation. 



Before leaving this question I would like to draw attention to 

 some effects of the cold, effects which make for extremely rapid 

 changes in the physical geography while the cold lasts. These 

 changes are far more rapid than any we are accustomed to under 

 present conditions. I do not wish it to be thought that there is any 

 necessity for periods of millions of years in order to allow time for 

 the changes such as we have suggested that man saw near 

 Bournemouth. 



We now see the Hampshire and Dorset rivers flow gently 

 down their slightly inclined beds, even in flood times not greatly 

 eroding their channels or widening' their valleys. We see also 

 that the waves of the sea only cause encroachments averaging a 

 few inches each year. We naturally think at this rate many 

 years would show no great change. But even at the present slow 

 rate of erosion the breach into the Solent Valley opposite 

 Bournemouth cannot have been made at any very remote period. 

 It is difficult to get a satisfactory estimate of the average rate at 

 which the cliffs are receding, but taking it at six inches annually 

 we find that the chalk hills of Dorset and the Isle of Wight would 

 be continuous across Bournemouth Bay about 50,000 years ago — 

 and this, geologically, is quite recent. However, we cannot get at 

 dates in this rough and ready way, and there are various other 

 things to be taken into account before we can give a date to this 

 great change. 



Amongst the things which must influence the rate of valley- 

 erosion, and the rate of the cutting' back of the sea-cliffs, the 

 first place must be given to climatic changes. With an Arctic 



* Principles of Geology, 12th edit., Vol. I., plate 4. 



