River Gravels and Boulder Clay. 533 



find Glacial deposits down to the level of the sea and 

 passing into it ; and near Romford, east of London, 

 there are tablelands covered with Boulder-clay, which'* 

 overlook the valley of the Thames. These phenomena, 

 taken as a whole, certainly show that all the upper 

 valley of the Thames is of older date than the Glacial 

 epoch, and though Boulder-beds are found at Southend, 

 on the north side of the mouth of the estuary, none oc- 

 curs on its southern shores, nor in the plains and valleys 

 of the Weald. Therefore, I now~see no reason why the 

 lower valley of the Thames west and east of London 

 should not be entirely pre-Glacial, in which case it may 

 be that some of its high-level gravel terraces belong to 

 that date. The question is still in debate among geo- 

 logists. I use the term high-level gravels to express 

 the fact that thick deposits of gravel and loams having 

 been formed in the valley, this alluvial detritus was 

 subsequently cut into a succession of river-terraces in 

 consequence of changes, slight but effective, in the phy- 

 sical geography of the area, and it is obvious that the 

 highest terrace overlooking the river must be the oldest, 

 and so on in succession till we reach the river-bank of 

 to-day. 



Before describing the relation of the river-gravels 

 of the south of England to the Glacial epoch and palae- 

 olithic implements and mammalia, it is desirable to ex- 

 plain some of the details of the manner in which rivers 

 have excavated their own valleys in solid rocks where 

 no valleys existed before the drainage of the country 

 took the general direction of its present flow. On the 

 Continent, the Moselle and the Seine form excellent 

 examples, and on a smaller scale many British rivers, 

 including the Thames, have followed the same law. 



Suppose a river flowing in a sinuous channel in the 



