22 LONDON DISTRICT. 



this position with annular outcrops of Thanet and Woolwich 

 Beds, indicating a dome-like structure. The Woolwich Beds 

 run from the surface outcrop near Camberwell through Rother- 

 hithe and the Isle of Dogs to Greenwich Marshes and thence into 

 Essex under Plaistow Marsh and East Ham Level, having been 

 proved at Beckton Gas Works. 



In the valley of the Lea is another dome, somewhat elongated, 

 bringing the Woolwich Beds up round Leyton; it extends from 

 Lea Bridge to Stratford and includes the freshwater bed which 

 has been detected in several excavations. The boundaries of 

 these outcrops will be shown on the New Series Maps, Sheets 256 

 and 270. 



The north-western outcrop (Sheet 1 of the map) extends 

 from near Radlett (where the Midland Railway leaves the area 

 of the map) south-westwards through Bushey to Moor Park, 

 south of Rickmansworth, and thence southwards along both 

 sides of the Colne Valley. The Reading Beds also form the 

 surface in the low-lying areas of Pinner, Northwood, Ruislip 

 and Ickenham. Local peculiarities are the large proportion of 

 sand in the strata and the replacement of the usual Bottom 

 Bed by a seam of pebbles 4 to 6 ft. thick. Blocks of sarsen and 

 of Hertfordshire puddingstone are abundant in the drift of this 

 district and have apparently been derived from the Reading 

 Beds; the puddingstone has been observed in situ towards the 

 top of the series near Radlett and elsewhere. Both rocks have 

 been formed by the deposition of secondary silica, cementing 

 a sand or a pebble bed respectively. The total thickness of the 

 formation is about 40 ft. at Bushey and Watford, But further 

 south, where the mottled clay becomes prominent, as much as 

 96 feet has been recorded at Hillingdon. 



At Battlersgreen in the north the following section has been 

 noted in several pits : 



Ft. 

 Drift gravel (irregular capping). 



Brown clay passing down into red, mottled and pale 



clay - ... - about 4 



White sand with layer of flint pebbles at base - about 2 

 Green sand - - about 1 



Bedded loam with a thin band of small flint pebbles 

 near top, passing down into bedded loams and sands 

 with occasional pebbles - at least 12 



Round Bushey there were formerly many good exposures; 

 the lower part of the series remains fairly constant, but the 

 mottled clay at the top increases rapidly to the south and south- 

 east from about 5 to 35 feet, with corresponding increase in the 

 total thickness. At Oxhey and Northwood however the sand 

 is predominant and the Bottom Bed is seen in Moor Park to 

 consist of pebbles in a sandy matrix to a thickness of 6 ft. The 

 Brick and Cement Works at Harefield reveal a complete section 

 as follows : 



Reading 

 Beds. 



