1870.] WOOD AVEALD-TALLET DENUDAXION. 19 



Essex *, and the Canterbury-heiglits drainage-areas, because a similar 

 intermingling must have resulted in these cases. Nevertheless, look- 

 ing to the fact that there is an absolute absence of gravel or brickearth 

 in the valleys of the Crouch (notwithstanding its tributary rivulets 

 extend up to the heights capped with glacial beds, which would have 

 supplied some gravel material) and of the estuary of the Thames, 

 which, as already described, have been excavated at a late period 

 through the high ridge separating the Thames from the East-Essex 

 gravel-sheet, and through those sheets themselves — and looking to 

 the feebleness of the gravel conditions exhibited by the wide sheet 

 skirting the Medway and Beult, to which allusion has been already 

 made, it is probable that by the period of this reversal the condi- 

 tions giving rise to the formation of brickearth and to the transport 

 of gravel by such flat-falling streams as the Thames and Medway 

 had ceased in a great degree, and given place to those different con- 

 ditions to which the river-mud or modern alluvium is due ; so that, 

 save to the extent of rearrangement by the action of the river- 

 waters, when more voluminous and at higher levels than now, most 

 of the material of the gravels along the lower levels of the Medway 

 and Stour valleys, except the sheets skirting these rivers and their 

 tributaries within the Weald, was probably transported before the 

 reversal took place. In the case of the Darent, inasmuch as its rever- 

 sal seems to have preceded the retreat of the sea within the Lower 

 Greensand escarpment, its gravels would be much more due to the 

 rivers while flowing in their present direction than would those of the 

 Stour and Medway, because at this earlier stage the conditions giving 

 rise to gravel and brickearth had not so nearly passed away; 



It may be asked where, if it be not represented in the gravels lying 

 without the Weald, has the debris of the subcretaceous rocks re- 

 moved to form the valley of the Weald gone ? The most probable 

 answer seems to me to be that it is distributed over the bottom of 

 the English Channel — not in the modern superficial shiugle, but in 

 the form of thick beds far out to the west covered by the modern 

 shingle, and concealed by it and by the waters of the Channel. 



To sum up the case as I have endeavoured to put it, we have the 

 following propositions : — 



1st. The absence from the Glacial beds of Essex of any debris 

 representing a considerable denudation of the Weald during the 

 Glacial period, and grounds, in the position and constitution of the 

 Boulder-clay of the Essex heights, for regarding the Wealden area 

 as beneath the sea during the accumulation of that clay. 



2nd. An absence from the principal Postglacial gravel sheets 



outside the north of the Weald of any quantity of Lower-Cretaceous 



or Hastings-sand material, adequate to represent the Postgla-cial de- 



- nudation of that valley by any agent that involves a transport of 



the material removed into the area occupied by these gravels. 



* That is to say, the portion only which occupies the Medway valley between 

 Chatham and the Nore ; the portion on the north side of the Thames, viz. in 

 East Essex, would, unless the North Sea at this time still remained at some 

 distance from the Thames mouth, be exempt from this later intermixture. 



C2 



