Page:VCH Hertfordshire 1.djvu/37

 GEOLOGY Period Formation Character of the strata Approximate thickness in feet Alluvium Peat clay loam etc I to I O Recent Valley gravels .... Gravels of existing rivers .... 5 to 20 River Drift Older river-gravel and sand. 5 to 'Jo Clay-with-flints. . . Brickearth .... Reddish clay with angular flints Loamy and sandy clay .... lu yj i to 25 c to 60 Pleistocene Chalky boulder-clay i o to 4.0 Glacial Drift. . . .| Gravel and sand I O tO 20 Westleton Shingle. . . Pebbly gravel I to IO Lower London Clay .... Brown or bluish clay, with base- ment-bed of brown clay and pebbles 20 to 1 70 Eocene Reading Beds .... Sands, mottled and plastic clays, and pebbles 25 to 40 Thanet Sands .... Grey and black sand o to i o Upper Chalk .... Soft white chalk, t with layers of flints about ^oo ' Chalk rock very hard, cream- coloured I to 4. Upper Cretaceous Middle Chalk. . . .- Lower Chalk. Upper Greensand. White chalk, with few flints Melbourn rock hard, nodular. Grey and white chalk .... Totternhoe stone hard, white. Chalk marl grey marly chalk. Soft marly sandstone 200 to 350 IO 65 to 90 6 to 12 2O to 60 Gault Stiff blue clay. Lower Cretaceous Lower Greensand. Iron-sands and ' carstone "... o to 10 Upper Jurassic Purbeck Beds .... Portland Beds .... Kttneridge Clay .... Clays and argillaceous limestone Sands and shelly limestone . Dark-coloured clay

Devonian Upper Devonian Dull-purple shale Silurian Wenlock Shale and limestone ....

The existence of Devonian and Silurian rocks at a great depth under the surface in this area is only known to us from borings made by the New River Company in the valley of the Lea. These very old rocks must for ages have formed a land-surface stretching right across Hert- fordshire, and probably giving rise to rivers flowing to the north and to the south. The shore-line of the Lower Greensand sea on the south apparently ran through the north of London, and that of the contem- poraneous northern sea through the north of Hertfordshire, trending north-east and south-west, and communicating in that direction, through 3