1867.] WOOD SOUTH-EAST OP ENGLAND. 395 



tion to this structure in a paper published in the Philosophical Ma- 

 gazine*. 



The completion of this survey has not only confirmed me in this 

 opinion, but has led also to some results bearing upon the geogra- 

 phical conditions under which the Thames gravel was accumulated, 

 and upon those which succeeded its deposition. These results have 

 removed from my mind much of the perplexity besetting the Post- 

 glacial structure of the portion of England embraced in this paper. 



In order to place in an intelligible form the evidence upon which 

 these results are founded, carefully prepared copies of both the small- 

 scale examination mapping, and of the large-scale survey made on 

 sheets 1 and 2 are necessary, as well as copious sections illustrative 

 of both. These being beyond the scope of a communication to the 

 Society, the only plan that has occurred to me by which the views 

 at which I have arrived and the evidence upon which they are based 

 could be recorded was to embody them in a manuscript memoir and 

 essay, with maps of the surveys I had made, to offer it to the 

 Library of the Geological Society, where it might be available for 

 the examination of those who thought the subject worthy of an 

 investigation, and to lay a brief epitome of the principal results 

 before the Society. This course I have adopted. 



Sheet 1, thus coloured geologically, shows that the Boulder-clay 

 of the Essex heights (which is the formation I have elsewhere had 

 occasion to describe as the Upper Drift, but which it will be more 

 convenient, by the way of contradistinction to the Postglacial 

 series, to caU the Upper Glacial) exhibits no trace whatever 

 in this part of having formed their southern edge by original 

 deposition, but the reverse. In all respects it presents the same 

 pelagic features as it does along its northern margin, where it is cut off 

 by denudation through central Lincolnshire, Huntingdon, Northamp- 

 ton, and Leicestershire — -and along its south-western margin in Buck- 

 inghamshire, where it is similarly cut off by denudation. Even in 

 the detached tracts occurring through South and Central Lincoln- 

 shire, and as far away as the small outliers north of Gainsborough, 

 the material of the deposit is so identical with that on the brow 

 of the Thames vaUey, that a basket of clay taken from either 

 extremity of this area could not be distinguished, although these 

 extremities are 140 miles apart. Therefore its abrupt termination 

 on the northern brow of the Thames vaUey, with the Tertiary 

 beds of this part complete beneath it, is to be ascribed to denudation 

 only. The position of the Upper Glacial clay relatively to the 

 Middle-drift formation (which, for a similar reason to that before 

 given, it wiU be more distinctive to call the " Middle Glacial ") is 

 one of overlap ; and this, as well as the position of both relatively 

 to the valley of the Thames, will be best shown by the following 

 diagrams illustrating the structure within Ordnance sheet No. 1. 



Fig. 1 shows the period when the Middle Glacial sea had, after 

 depositing its sediments, eaten its way by coast-erosion between 

 either islands or promontories formed of the upper part of the Lon- 

 * 4th series, vol. xxvii. pp. 180-190. 



VOL. XXIII. PAET I. 2 E 



