Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/101

 land, had that region been under subaerial conditions during the prevalence of this icy envelope ; yet, after allowing for the excessive denudation which has, as it seems to me, prevailed in postglacial times over the south-east of England, the complete absence of the smallest vestige of any such accumulation as we find in Lincolnshire is, I think, reconcilable only with the conclusion that during the formation of the Boulder-clay this region was covered by the sea — a conclusion, moreover, to which the position of the Boulder-clay at elevations of 300 feet and upwards on the Essex heights that front the North Downs equally points.

So far, therefore, as the evidence of the detrital beds of Glacial age lying without the north-eastern part of the Weald affords a test, we are, I think, entitled to infer that the Weald was not during the climax of the Glacial period an area undergoing denudation by streams either of water or of ice, and, indeed, that, with the exception of the earlier part of that period, it was not above water at all.

Passing now to the Postglacial beds, the principal formations of this age lying without the Weald are the gravels to which I have already made allusion under the names of the gravels of the Thames, of East Essex, and of the Canterbury heights.

Precisely the same kind of reasoning is applicable to these as to the case of the Glacial beds, so far as concerns the debris of sub- cretaceous rocks. If the Stour, the Med way, and the Darent, running outwards from the Weald, had effected any thing like the prodigious denudation attributed to them, fragments of the stonebeds of the Lower Greensand ought to make up at least half the volume of the Thames, the East-Essex, and the Canterbury-heights gravels where these three streams pass through them.

In the Thames gravel of this part, however, such fragments, though common, form but a small proportion of the gravel mass, the bulk of which is flint with some quartzites intermixed. The East- Essex gravel, both where it lies within the valley of the Medway between the Wore and Rochester, and where it extends along the east coast of Essex, presents similar features ; while the gravel of the Canterbury heights, which forms the sides of the valley through which the Stour flows, is even more exclusively flint in its composition, as it requires a search of some time to find half a dozen fragments of any other material, so that in this gravel the proportion of any other material than flint is probably not 1/1000 of the mass.

If we reflect how small is the elevation of these gravels above the streams which flow beneath them, in comparison with the elevations which the subcretaceous rocks attain within the Weald, can it be contended that gravels so composed could have been deposited from rivers which were effecting the enormous denudation that has placed these rocks as they now are ? Can we, even if we reject the hypothesis of this great fluviatile denudation, reconcile the composition of these gravels with their deposition from these rivers when in greater volume than now ?

The answer seems to me to be clearly negative, and that under such circumstances the flint in the East-Essex gravel between Rochester