200 THE GEOI.OOV OF THE LEA VALLEY. 



of gravel which has been brought down in the channel of the stream 

 and deposited where the current was sluggish. Now and then, 

 during floods, inundation mud would settle down here and there 

 above the gravel, and remain as patches of brick-earth. And at a 

 later date the river, flowing at a slightly lower level, and being more 

 sluggish than at an earlier period, has ceased to bring down gravel 

 in its channel, but has deposited instead the alluvium of the marshes 

 through which it now meanders. We may now consider the evidence 

 as to the lateral shiftings of the Lea. 



In the case of the Thames a glance at the maps of the Geological 

 Survey shows that from Windsor downwards it now takes on the 

 whole a more southerly course than it once did. This is made 

 manifest by the very much greater breadth of river deposits north 

 than south of the river both above and below London. As regards 

 the Lea, we find that from its junction with the Stort downwards it 

 once flowed one, two or even three miles west of its present channel, 

 while it never ran, on the whole, in its earliest days much further 

 eastward than it now does. An interesting practical result of this 

 is that we find a series of old populous villages from Tottenham to 

 Hoddesdon, on the old river-gravel west of the Lea, along the 

 course taken by John Gilpin in his involuntary pilgrimage to Ware. 

 But on the Essex side of the Lea opposite there is no large ancient 

 village except around Waltham Abbey, where there is a patch of 

 old river-gravel of exceptional size. For the London Clay, which 

 usually bounds the marshes on the Essex side, is incapable of 

 affording any water supply, while the old river gravel at Waltham 

 Abbey, Cheshunt, Edmonton, and elsewhere, allows water to per- 

 colate through it, and the underlying London Clay prevents it 

 from sinking below a moderate depth, at which it can be utilised 

 by means of a pump. In the days before a knowledge of geology 

 caused deep borings to be made and companies to be formed for the 

 supply of water thus obtained, the power of procuring water from 

 surface gravel was a chief influence in the determination of sites for 

 villages and even for towns. 



Above the junction of the Lea and Stort, the Lea and its 

 tributaries flow in valleys the sides of which are usually capped 

 by Boulder Clay. South of the junction Boulder Clay appears 

 only here and there in outlying patches on the higher ground. It is 

 almost perfectly certain, however, that Boulder Clay once spread 

 over the site of the valley of the Lea many miles below the junction 



