Wood— Structure of Valleys in Essex. 351 



gravel to the Upper and Middle Drift and the great flexure by which 

 this valley originated. It will be observed, from the features exhi- 

 bited by this section, that the position now occupied by the East Essex 

 gravel cannot have been that in which, relatively to the Upper and 

 Middle Drift, it was deposited, as it occupies a higher level than the 

 nearest part of the Middle Drift, which descends to near the water's 

 edge on one side of the Blackwater estuary, but is wholly absent 

 on the other. The cause of this feature exists in the great dis- 

 turbances which have succeeded the deposit of the East Essex gravel, 

 and which I will endeavour to explain. By the Map, and by 

 Section 9, it will be seen that the Middle Drift extends to the very 

 bottom of the upper (or Kiver) valley of the Blackwater, forming 

 the whole of the lower level of that valley; on the north side of the 

 Chelmer it also descends to near the edge of the river, standing 

 at but little elevation above it. Very nearly the same thing takes 

 place on the north side of the lower or estuary valley of the 

 Blackwater ; whereas on the south side of the valley of this estuary, 

 as well as on the south side of the Chelmer river, the Middle Drift 

 does not come within a considerable distance of the water, and 

 stands at great elevations (from 100 feet upwards) above it. In 

 short, the Chelmer Eiver and Blackwater Estuary form a dividing 

 line, on the south side of which the entire block of country, lying 

 between this river and estuary and the mouth of the Thames, has been 

 elevated without any corresponding elevatory movement having taken 

 place on the north side, but rather the reverse. The disturbances 

 which produced this detachment and local elevation have been accom- 

 panied, I will endeavour to show, by the re-excavation or second 

 denudation of the coim^try forming the south side of the Blackwater 

 estuary and upper valley of the Crouch. 



Section 9 will show that the gravel on the north side of the 

 Estuary belongs to the Middle Drift, and not to the East Essex gravel, 

 and also the extent to which the great flexure, emanating from 

 the Canterbury centre, brought into existence the original valley of 

 the Blackwater ; for it will be seen that the sand and gravel (e) on 

 the north-west flank of the flexure passes under the Boulder-clay 

 (/), coming out both above and below that clay, thus proving its 

 identity with the Middle Drift ; and this gravel may be traced with- 

 out a break over the summit of the flexure in one part down to 

 the north shore of the Blackwater estuary. ^ This great flexure or 

 rolling earthquake surge passes but with diminishing force through 

 the Upper and (where that formation is present) through the Middle 

 Drift also, nearly as far as the vale of Belvoir in Rutlandshire, 

 forming in its way several troughs of denudation, of which the 

 most marked are the great troughs excavated between the Upper 



^ The line of section has been chosen in order to show the Middle Drift continuous 

 over the crest of the ridge. If, however, the line of section were drawn a little more 

 to the north-east, all the crest of the ridge would appear denuded of the Middle 

 Drift and exclusively occupied by the London clay ; e.g. if the section were dra wn 

 from Inworth to Bradwell-on-Sea this would be the feature of the section, and the 

 London clay on the crest of the ridge would be capped by summit gravel {x 1). 



