3o4 Wood — Structure of Valleys in Essex. 



that require the explanation of sticli a subsequent disturbance to 

 reconcile them. 



Section 10, drawn across the rectilinear lower valley of the 

 Crouch, shows the East Essex gravel cut through by this rectilinear 

 valley ; while Section 11 is drawn across the same valley a little 

 higher up, where it has cut through the London-clay side of the 

 more ancient valley which contained the East Essex gravel. The 

 low level occupied by that gravel in Section 10 should be con- 

 trasted with the far higher gTOund of London clay, through which 

 the Crouch cuts in Section 11, in order that it may be realized that 

 the East Essex gi'avel, although it is thus cut through by the alto- 

 gether independent and much newer valley of the Croxich, is as truly 

 the deposit of a valley hereabout (save that the eastern slope of it 

 has now disappeared into the North Sea) as it is more to the south, 

 where it occupies the lower valley of the Medway . Section 13 has been 

 added to show the structure of the upper valley of the Crouch, which, 

 it will be remembered, is part of the one original great trough that 

 was afterwards re-excavated in part by the Crouch in its estuarine 

 condition, and in the rest by the more-extended estuary of the Black- 

 water, and divided between them. From the section it will be seen 

 that this original trough is cut down from the Boulder-clay on the 

 western side, but that on the other the Bagshot sand and London clay, 

 only, form the valley side. This is due to the peculiar structure of 

 the Upper Drift over the south of Essex, that formation having, at its 

 commencement, eroded the Bagshot sand and taken its place, but 

 afterwards, by submergence of the land, overspread it ; so as in one 

 place to lie imder a brow of Bagshot sand, and in another to rest upon 

 it, in that case occupying a much higher level than in the other place. 

 The post-glacial denudation which gave rise to the valley, has, on 

 the east of the section, removed the Boulder-clay and left the Bagshot 

 sand ; while on the west of the section, it has partially spared the 

 Boulder-clay, the sea of which had long anterior to this removed 

 the Bagshot in that part. It is to this structure that the Boulder- 

 clay does not remain on the summit of the eminences crowned by Bag- 

 shot sand, which form Langdon, Thimdersley, and Hockley Hills, 

 the Isle of Sheppey, and Hampstead and Highgate Hills, while 

 traces do remain on the similar eminences forming the Brentwood 

 plateau, and the Havering and Epping Hills. 



In the next number of the Magazine, I projDOse to consider the 

 relation which the East Essex Gravel bears to the Structure of the 

 Weald Valley. 



IV. — On the DisiNTEanATioN OF A Chalk Cliff. 

 By the Eev. Osmond Fisher, M.A., F.G.S. 



AS a slight contribution to the elucidation of questions of denuda- 

 tion, and at the same time an exemplification of the appli- 

 cation of mathematics to a geological problem, I send the following : 



