144 GEOLOGY OF THE DISTRICT AROUND DAGENHAM BREACH. 



the Thanet Sand and Woolwich Series, with, here and there, some 

 Blackheath Pebble Beds, and even a little London Clay. 



We may now consider briefly the general geological structure of 

 the district around us and the way in which its present scenery has 

 been slowly evolved. The lowest formation, the Chalk, dips north- 

 ward from its escarpment, along the line of the North Downs, beneath 

 the Tertiary Beds of the London Basin, and rises again from beneath 

 them in Hertfordshire and north-western Essex. But in addition to 

 this general arrangement there is a slight anticlinal curve along the 

 valley of the Thames which raises the Chalk to a higher level than it 

 would otherwise possess there. In consequence of this anticlinal the 

 quarries at the base of the Lower Tertiary escarpment opposite, and 

 those at Greenhithe, Gravesend, and Cliffe, show the Chalk lying 

 either level or with a slight southerly dip instead of a northerly one, 

 the Tertiary Beds above necessarily partaking in this upturn. On the 

 Essex shore the northward dip is speedily resumed, as shown between 

 West Thurrock and the Mardyke, along the course of the new 

 railway ; the lowest formation seen at the surface over the greater 

 part of Essex being the London Clay. Two lines of fault, also, 

 though they have not greatly influenced the scenery, have had much 

 to do with the variations in the nature of the beds underlying the 

 river-deposits. One of these faults, which has a downthrow at 

 Greenwich of about loo feet to the north-west, ranges from St. 

 John's railway station, along the northern margin of Greenwich Park, 

 and may pass under the Thames a little west of Woolwich Dockyard. 

 If produced so as to cross the Albert Dock, its presence may perhaps 

 account for the fact that Chalk is found below the river-beds at the 

 eastern end of the dock and Woolwich Beds at the centre and west. 

 Another line of fault ranges nearly east and west at the foot of the 

 Lower Tertiary escarpment between Woolwich and Erith, having a 

 downthrow to the north. Hence the Lower Tertiary Beds are brought 

 in again north of this line of fault and found beneath the river- 

 deposits of Erith marshes. 



But the chief influence in giving its present aspect to the scenery 

 around has been the Thames. Beginning as a much more powerful 

 stream, flowing at a much higher level, it once ran by Romford, 

 Hornchurch, and North Ockendon, and thence by Stiflbrd to Stan- 

 ford-le-hope. It has since been occupied in deepening its valley and 

 in taking a more southerly course. No doubt it once flowed at the 

 foot of the Lower Tertiary escarpment, from which it is now sepa- 



