Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/113

 Essex *, and the Canterbury-heights drainage-areas, because a similar intermingling must have resulted in these eases. Nevertheless, looking 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 conditions 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 conditions 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 reversal 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 removed 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 shingle, 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 Postglacial denudation of that valley by any agent that involves a transport of the material removed into the area occupied by these gravels.

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

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