GEOLOGY 



than the age of that formation, for it is also present when sands of the 

 Reading Beds rest upon the Chalk, as in the Bushey chalk-pit near 

 Watford. 



When fully developed, as in Kent, the Thanet Sands are 50 or 60 

 feet thick, but they thin out under London to 20 feet, and are only 

 known to occur in Hertfordshire from their presence in the Cheshunt 

 boring, where their thickness is reduced to about 10 feet and they consist 

 of grey and black sand. 1 They are of marine origin. 



South-east of a line preserving a general north-east and south-west 

 trend, but very irregular, crossing the Lea and Colne districts from a 

 point about half a mile south of Stocking Pelham near Bishop Stortford 

 to Woodcock Hill near Rickmansworth, the Chalk is overlaid by the 

 Reading Beds and London Clay, the escarpment of which follows, at a 

 distance varying from less than a quarter of a mile to a mile and a half, 

 the river Ash downwards from Furneaux Pelham to Amwell Magna, the 

 river Lea upwards from Hoddesdon to Hatfield, and the river Colne 

 downwards from North Mimms to Harefield. Of the Reading Beds 

 there is normally a narrow outcrop along this line, wider in the east than 

 in the west owing to the difference in the slope of the ground ; and the 

 London Clay reposes upon them, forming a range of hills along its 

 escarpment generally from about 300 to 400 feet in height, and, at its 

 highest point, Stanmore Common, between Watford and Elstree, rising 

 to 500 feet. 



The Reading Beds are represented in Hertfordshire by a very 

 variable series of sands, mottled clays, and pebble-beds, there usually 

 being at their junction with the Chalk the layer of green-coated flints 

 already mentioned. They are here of estuarine origin, thus differing 

 from all the formations already considered, which are of marine origin. 

 From their small thickness, which varies from about 25 to 40 feet, and 

 the usually rather steep slope of the ground at their outcrop, they do not 

 occupy any great extent of country, and in most places have but little 

 effect upon the surface-soil. Where their sands and clays predominate 

 and get mixed with the London Clay, the soil is usually fertile, but 

 where their beds of rounded flint-pebbles are much developed, as in the 

 neighbourhood of Hatfield and North Mimms, the soil is particularly 

 sterile. At Radlett and near North Mimms the principal pebble-bed is 

 consolidated by a silicious cement into a conglomerate, well known as the 

 Hertfordshire conglomerate or ' plum-pudding stone.' Although it is 

 only known to occur with certainty in situ in this part of Hertfordshire 

 at the present time, it has probably at some former period had a much 

 greater extent, for masses of the conglomerate are strewn here and there 

 nearly all over the county, and are also found beyond it. In a gravel-pit 

 north of St. Albans there are large unwaterworn masses of it apparently 

 but little disturbed from their original position, for they seem to form 

 part of too extensive a bed to have been shifted horizontally ; indeed, in 



1 Whitaker and Jukes-Browne, ' On Deep Borings,' etc., Quart. Journ. Geol. Soc., vol. 1. 

 (50), p. 508 (1894). 



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