414 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [JunC 19, 



and of the position occupied by the implement-gravels in the latter, 

 to which I hav^e just referred. The case of the Thames river is, that 

 east of London it has been introduced at a recent period over a Post- 

 glacial land-surface, represented by a forest which grew upon the 

 gravels of the series x 4 and cc 5. The forest-bed is a peat almost 

 exclusively composed of the twigs and leaves of trees, in which the 

 tree-trunks generally lie flat in immense numbers. At the base of the 

 peat, however, I have observed stools in one or two instances rooted 

 into the gravel. This was in the southern outfall sewer, about 20 

 feet below the level of the present high-water mark. There is clear 

 evidence, in the universal occurrence of this bed under the marshes 

 between the mouth of the Lea and Erith, that the entire bottom of 

 the Thames valley in this part was occupied by the forest, so that 

 the Thames could obtain no access to the North Sea but by passing 

 over it : and this is what it has done; for the marsh-clay, which repre- 

 sents the deposit of the Thames before embankment, has covered 

 the forest for a thickness of from 3 to 8 feet, and towards the upper 

 valley of the Lea has overlapped it, so as to rest on the gravel near 

 Tottenham, while the channel into which, by embankment, the 

 river has been confined, cuts through it, so that the peat and the trees 

 come out at low water on either bank. Nothing of the sort, how- 

 ever, occurs in the vaUey of the Thames-mouth, which, at a period 

 apparently very closely coinciding vdth the introduction of the river, 

 was cut through the dividing ridge of the Thames and East-Essex 

 gravel-troughs. The Thames river thus occupies a position imply- 

 ing that relation only to the gravels associated with it which London 

 itself bears — namely, one of introduction at a late period ; and 

 it can hardly be unreasonable therefore to associate its origin with 

 that reversal which all the other features of the Postglacial period 

 conspire, as I contend, to prove. 



It will be found, on an examination of the region occupied by the 

 valley of the Thames-mouth and its allied valley of the Crouch river, 

 that a structure exists there which is very anomalous at first sight, 

 but which, if the views I have put forward and endeavoured to sub- 

 stantiate in the memoir are well founded, is altogether in harmony 

 with the other features displayed by the Thames river. We have 

 in it the evidence of a very recent and local depression and elevation, 

 by which the excavation of the valleys of the Crouch and Thames- 

 mouth, and the redenudation of the south side of the valley of the 

 Blackwater estuary, have been effected. The block of land thus locally 

 depressed and elevated extends from the south side of the Black- 

 water mouth to the Med way ; and I think also that the north coast 

 of Kent has more or less partaken of the same movement. The rede- 

 nudation of the south side of the Blackwater estuary-vallej^, and the 

 excavation of the valleys of the Crouch and Thames-mouth, have 

 been unaccompanied by even the feeblest trace of either gravel or 

 brick- earth ; and as there is not a valley, whose denudation we trace 

 to any part of the Postglacial period, as old as, or older than, the im- 

 plement-gravels, but yields some deposit of this sort, however insig- 

 nificant, we are naturally led to assign the excavation of the Thames- 



