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III.—On the Relation which the East Essex Gravel Bears to the Structure of the Weald Valley

Published online by Cambridge University Press:  01 May 2009

Searles V. Wood Jun
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Extract

In the former paper I adverted to the evidence which the Thames gravel afforded, that the channel in which it was deposited opened out over the Wealden area, and the corroborative evidence which the East Essex gravel furnished of a similar state of things.

Type
Original Articles
Information
Geological Magazine , Volume 3 , Issue 27 , September 1866 , pp. 398 - 406
Copyright
Copyright © Cambridge University Press 1866

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References

page 398 note 2 Geological Magazine, Vol. III., pp. 57 and 99.Google Scholar

page 399 note 1 In the mapping, by the Geological Survey, of the only portion of the East Essex gravel that has yet been published by them, that within two miles of Rochester, the gravel is made to overlap the junction of the London clay with the Woolwich beds at Cockham Wood Reach, whereas, it is really divided from the Woolwich beds there by a considerable thickness of London clay, and all three beds, including the gravel, should sweep round parallel to Cockham Wood Reach, and to the scarp of the Downs. The gravel is also not carried far enough in the Survey sheet to the west, there being several exposures beyond the boundary given to it there. The delineation of the gravelin the Survey sheet is so made as to appear at this part as though the chalk sides of the Weald Valley had been formed before the gravel came into existence. The corrected delineation at this part is given in an enlargement at the foot of the Map accompanying this paper. No distinction is of course made in the Survey Map between this gravel and those which I have grouped as several successive series.

page 400 note 1 This is the portion of Map that has been reduced from the Geological Survey Sheet. Whatever hesitation may be felt in admitting the accuracy of the part (that north of the parallel of Rochester) which has been reduced from my own survey, none, I presume, will be entertained as to this portion.

page 401 note 1 Restorations, such as are attempted in division 3 and 4, are regarded, and justly so, with much suspieion. I have only resorted here to that course in order to render my views of the progress of the great post-glacial denudation intelligible at a glance, when pages of description would fail in that object.

page 401 note 2 The attenuation of the Chalk oyer the Weald centre, and exposure of the upper part of the beds beneath it at this time, as also the possible erosion of a part of the Lower Tertiaries over the Weald during some part of the Middle Tertiary period, may be remembered here, for exactness sake, but it is not material to the argument.

page 402 note 1 Probably aided by the passage of shore-ice, as suggested by Mr. Brodie reference to the Somme Valley (Vol. III., p. 278, of this Mag.). In that view of Mr. Brodie, so far as it goes, I concur, as I regard the Somme Valley as contemporaneous with that of the Weald, but without the previous formation of an isthmus to cut off the North Sea tide, I cannot see how any more tidal rush could arise in the old trough of the Somme than now exists in the equally funnel-shaped mouth of the Thames River.Google Scholar

page 403 note 1 Of these ares cutting through the drift, the repetitions of the arcs forming the urved valleys (shown in Division 1 of the Map), the principal is that extending from leading to Hitchin. The symmetrical coincidence of this are, and the cut which between Hemel Hempstead and Royston) it makes through the upper and Middle Drift, may be seen in the small sketch Map in Division 3. The next great repetition the are Market Harboro, Rockingham, etc., which cuts in a similar way through he Upper Drift, capping the heights between those places.

page 403 note 2 I have carefully extracted them all from the French Ordnance Maps.

page 404 note 1 Mr. Prestwich takes a totally opposite view to that which I have been endeavouring in these papers to establish, for he says (Quart. Journ. Geol. Soc., vol.xxi. p. 441). “The old rivers of the Wealden area debounched, as do those of the present day, outwards into the Thames valley, but were of much greater size and extent.” In describing the Gravels of the Weald Vally, also, Messrs. Le Neve, Foster, and TopleyGoogle Scholar (Quart. Journ. Geol. Soc., vol. xxi. p. 464) expressed the decided opinion that the excavation of that valley between its Chalk escarpments had been due to the action of the Medway ‘flowing in the same direction as at present’ during the Glacial period, the Weald, they say (p.465) “not appearing to have been under water at all during that period.” That the latter gentlemen should have arrived at the opinion that the denudation forming the valley and giving rise to the gravels was caused by the Medway flowing in this direction, in the face of their own carefully drawn map of those gravels, seems strange Pressed by the difficulty that the chief part of the material composing the gravels of the Greensand terrace came from the north, these gentlemen have resorted to a small tributary of the Medway that in one part of its course apporaches the Tertiary area, as the source from which the principal part of the material of these gravels was derived; and from the direction of the green line in the map accompaning the “Theoretical Considerations” of Mr. Prestwich, in the Philosophical Transactions, part 2, 1864, that gentleman, it would seem, takes a similar view of the origin of the cretaceous débris. With respect to the converse of this proposition, i.e. the presence of pieces of Wealden sandstone in these gravels, there seems no reason why it should not have arisen by shore ice transport from islands which had, at this period, come into existence in those parts of the Hastings sand country that are most elevated. The occasional occurrence of land and freshwater shells in some of these gravels is not conclusiveevidence that the gravels were of freshwater origin, as we find these shells intermingled with such saltwater forms as Scrobicularia piperata and Tellina solidula in the Claeton bed; but when we consider that these gravels were formed at the mouth of a river, and reflect how likely it is that a closed sea, such as that of which I have been supposing the Weald Valley to have formed the head, should, under subarctic conditions, have approached (except in the existence of a tide) the brackish condition of the present, Baltic, the obstacle to the marine theory, arising from the presence of freshwater shells, does not appear a very serious one. The formation of gravel at all during the latter part of the period in the Weald may, indeed, have been due to this freshwater condition permitting the formation of ice.Google Scholar

page 405 note 1 Archæologia, Vol. xxxviii., 1860.Google Scholar