72 LONDON DISTRICT. 



Island is now a tract of Alluvium between Hornchurch and 

 Rainham Marshes, and may have been a marshy island. 



The Alluvium consists of alternations of silt or mud, clay, 

 shell-marl, peat, sand, and (usually at the base) gravel. In 

 places along the Brent Valley, near Greenford, it is largely made 

 up of re-deposited London Clay with seams of gravel in it. 

 Vivianite (phosphate of iron) is occasionally met with in the 

 alluvial deposits. The ' marsh-clay,' described by Mr. Whitaker 

 as ' an impure and often sandy clay, more or less charged with 

 vegetable matter,' may be said to include both freshwater and 

 tidal alluvium. 



The present course of the Thames bears no necessary relation 

 to the full depth of Alluvium ; masses of Lower Eocene strata 

 have been dislodged from the bed in dredging operations below 

 the Tower Bridge, and it is probable that the Chalk is not far 

 below the mud of the river-bed in some of the reaches between 

 Woolwich and Greenhithe. Spurrell, however, remarked that 

 ' notwithstanding that the waterway of the Thames is very 

 irregular, it is clear that it has kept its present line of flow the 

 same, within narrow limits, since it first became estuarine.' 1 



The depths of Alluvium, apart from underlying gravel, in 

 London bordering the Thames from Battersea to Deptford are 

 from 7 to 16 ft. ; the depth at Shadwell is 16, at Walbrook 25, 

 and in the Isle of Dogs from 8 to about 20 ft. Eastwards there 

 is 35 ft. or more at East Greenwich and Charlton, from 26 to 31 

 in the marshes of Plumstead and Erith, 22 at Crossness, and up 

 to 50 ft. further down the estuary. 



On the northern side of the river at Plaistow and East Ham 

 the depth is from 18 to 30 ft., at Dagenham (locally) 16, and at 

 Aveley Marsh 30 ft. Further on beyond our region as much as 

 54 ft. of mud, clay and peat were proved at Tilbury. In the 

 records of borings two or three layers of peat are sometimes met 

 with, alternating with silt or mud and clay, and often a peaty 

 bed is noted at the base, sometimes with remains of trees. 



That the alluvial deposits are newer than the Cave Period 

 has already been mentioned ; but since the implements of Les 

 Eyzies type found on the gravel surface beneath them are unworn, 

 the alluvium at those localities must have been laid down shortly 

 after that stage ; on the other hand, much of the alluvial mud 

 found below London is of post-Roman date. The Holocene or 

 Recent division thus covers the Neolithic or Newer Stone Age, 

 the Bronze Age, regarded as extending from 1800 to 600 B.C., 

 and the Iron Age which merges into historic times. 



Of neolithic times, Clement Reid remarks that ' at first the 

 land stood at an elevation of some 60 or 70 ft. above its present 

 level, so that many of the river valleys were cut to that depth 

 below the sea, and much of the English Coast was fringed with 

 a broad strip of alluvium, which probably almost connected our 

 island with Belgium and France. The climate during this epoch 



l Proc.-Oeol. Assoc., vol. xi, 1889, p. 212. 



