Page:VCH Hertfordshire 1.djvu/38

 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