I02 NATURAL SCIENCE. August, 



former streams, for beds of gravel lie along them. Sections in the 

 gravels on the floors of the dry Chalk valleys are scarce and 

 unsatisfactory ; there is a small one within the fork of a lane and the 

 main road from Missenden to Prince's Risboro', just to the north-east 

 of Hampden House. The hole shows a coarse gravel of unrolled 

 flints with but little sand and without any oolitic material. Mr. 

 Whitaker records a note by Mr. Jukes Browne upon the gravels at 

 Saunderton in the next valley to the west, which show the same 

 characters (i, p. 447.) 



The nature of these gravels is important, as it is to their con- 

 stitution that we have to trust to know what was the former course 

 of the rivers that eroded the valleys in which they occur. There are 

 two possible explanations : either these streams rose on the Oolites 

 to the north, and are earlier than the Chalk escarpment, or they were 

 confined to the Chalk (as the Chess still is), and existed when this 

 formation extended much further to the north-west than at present. 

 That this latter view is correct appears the more probable, as the 

 gravels of these dry valleys are composed solely of material derived 

 from the Chalk and the " pink " gravels which overlie it. If the 

 head-waters of the rivers had been situated on Oolitic rocks, it is 

 about certain that some fragments of these would occur in the gravels. 

 It might be suggested that these have been eliminated owing to the 

 distance of the valleys from the Oolites ; but when we remember that 

 fragments of these rocks occur in the Thames gravels on the south 

 side of the Chalk, and much further from the Oolites than the 

 Saunderton — West Wycombe valley, this explanation seems insufii- 

 cient. That the valleys are later than the so-called "Glacial 

 Gravels " or Newer Plateau Gravels, we have already seen ; but the 

 very irregular nature of the floor of the valley, from which all traces 

 of any former channel have been destroyed, shows that it is of some 

 considerable age. To learn whether the valleys were earlier or later 

 than the Boulder Clay, we must turn to a member of the series 

 which cuts this deposit. The Lea serves our purpose very well ; 

 in its upper course it presents identical features with those of the 

 Miss and Chess, while we know that at Hertford the Lea Valley was 

 eroded in the period that intervened between those of the Newer 

 Plateau Gravels and the Boulder Clay, deposits of the latter of which 

 occur within it. If therefore, as seems highly probable, all the 

 north-west to south-east valleys through the Chilterns were formed 

 simultaneously, then the Chalk escarpment was breached in the 

 period between the formation of the Newer Plateau Gravels and the 

 Boulder Clay. 



The next point for consideration is as to the evidence to show the 

 age of the escarpment itself. The distribution of the Boulder Clay 

 gives us a good hint in this direction. This deposit was apparently 

 formed by the southernmost portion of the East Anglian ice-sheet, which 

 travelled from north-east to south-west : towards its end it appears to 



