404 Wood— On the East Essex Gravels. 



irresistibly to the extension of the Boulder-clay sea over the north 

 of France, and to the denudation which I have traced having been 

 the result of the emergence of the country from that sea, greatly pro- 

 longed in some parts, and accompanied by much subterranean dis- 

 turbance.^ The importance of this, the Post-glacial period, has yet 

 to be realized, and the length of its duration, marked rather by the 

 amount of deposits denuded than by those formed in it, is, I believe, 

 scarcely imagiaed. The Upper Drift or great Boulder-clay of the 

 eastern and central counties of England (for there is one and possibly 

 even two Boulder-clays older, as well as, at least, one that is newer 

 in these counties), bears, wherever it rests on strata newer than the 

 Trias south of Flamborough Head, evidence of having been affected 

 by a series of disturbances, apparently simultaneous, and taking this 

 curvilinear form, which were followed by its elevation from the sea. 

 This disturbance, or physical break, I regard as a natural dividing 

 line between the glacial and post-glacial periods ; all the beds termed 

 post-glacial occupying, over this area, a position that is, in effect, un- 

 conformable to the Upper Drift, although they be, for the most part, 

 not in contact with it. On the older side of this dividing liae, the 

 disturbances, or imconformity between the successive formations, are 

 insignificant ; while on the newer, or post-glacial side, they have 



^ Mr. Prestwich takes a totally opposite view to that which I have been endea- 

 vouring in these papers to establish, for he says (Quart. Journ. Geol. Soc, vol. sxi. 

 p. 441), "The old rivers of the "Wealden area debouched, 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 Valley, also, Messrs. Le Neve, Foster, and 

 Topley (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 '■floxuing 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 approaches the Tei'tiary 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 accompanying 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 debris. 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 conclusive evidence that the gravels 

 were of freshwater origin, as we find these shells intermingled with such saltwater 

 forms as Scrobicidaria jjipcrata and Tellina solidula in the Clacton 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 per- 

 mitting the formation of ice. 



