\ ClolJuJl 



' Benningtun 



A HISTORY OF HERTFORDSHIRE 



Oxfordshire, with the southern sea. Silurian rocks formed the highest 

 land. At a depth of 797 feet beneath the surface (685 feet below Ord- 

 nance datum) the Wenlock Shale with bands of limestone was found 

 at the boring between Hertford and Ware, where is now the Broad- 

 mead well, dipping about i west of true south 

 ?at an angle of 41 with the horizon. 1 Upon 

 these Silurian rocks, after some earth-movements 

 had taken place, disturbing their original hori- 

 zonal position, Devonian rocks were deposited. 

 At a depth of 980 feet beneath the surface (872 

 feet below Ordnance datum) Upper Devonian 

 shale was met with at Turnford near Cheshunt, 

 a few miles south of the Ware boring, dipping 

 about 17 west of true south at an angle of 25 

 with the horizon. 2 Devonian rocks have also 

 been found under London at an increasing depth 

 as we proceed from north to south. They 

 were therefore deposited unconformably upon 

 the Silurian rocks, and the old land-surface grad- 

 ually became lower from Ware southwards. 3 

 South of London it was much lower, as shown 

 by the great depth to which the Netherfield 

 boring in Sussex was carried without reaching 

 it. 



After the deposition and upheaval of the 

 Devonian rocks a very long interval supervened 

 before Hertfordshire was undoubtedly again 

 beneath the sea, and considerable earth-move- 

 ments took place, as shown by the angle of dip 

 of these rocks. During this interval the whole 

 of the Carboniferous, Permian, and Triassic, and 

 nearly the whole of the Jurassic rocks were de- 

 posited in other parts of England ; at least if any 

 older rocks than the Upper Oolites ever existed 

 in our area, no trace of them has yet been found. 

 At Puttenham, in the extreme north-west 

 of the county, beyond the Tring reservoirs, near 

 the bottom of a bore-hole carried to a depth of 

 225 feet from the surface, the Kimeridge Clay 

 was met with. The well was carried to a depth 

 of 1 1 5 feet entirely through Gault clay, here 

 about 1 50 feet thick ; the boring was commenced in this clay and passed 



1 Francis, 'On the Dip of the Underground Palaeozoic Rocks at Ware and Cheshunt.' 

 Rep. Brit. Assoc. for 1895, p. 451 (1896). 



2 Op. cit. p. 452. 



8 ?x7 Pk ' n S n ' '. n the Recent Discov e<7 of Silurian Rocks in Hertfordshire,' Trans. 

 t^atfird Nat. Hist. Soc., vol. ii. p. 241, see pi. ii. (1880). 



4 



(Turnford 



