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 100 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 Stifford 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-