A HISTORY OF HERTFORDSHIRE 



Lea which extends from Hatfield to Hoddesdon on the south-east. 

 Into this inclined plain the valleys have been cut, for the hills of this 

 part of Hertfordshire ' are not ridges elevated above the general level of 

 the surface ; but appear to be such only when viewed from the valleys 

 of the rivers, whose waters have cut and furrowed deeply below the 

 general level.' 1 Here and there these rivers have cut through the super- 

 ficial deposits and the Upper Chalk into the Middle Chalk, exposing 

 the Chalk Rock, which may thus be seen in the Bulbourn Valley as far 

 south as Rough Down near Boxmoor. 



There are a few exceptions to the almost uniform slight dip of the 

 Chalk towards the south-east. South of Royston the dip is reversed, a 

 line of flexure having been traced for a distance of five miles along the 

 escarpment. In the Memoir on Sheet 47 of the Geological Survey 2 

 there are sketches of chalk-pits north of Barkway and on Reed Hill, 

 showing a dip at about the junction of the Middle and Upper Chalk 

 which gradually increases from zero to as much as 60 to the north. 

 This appears to be merely a local disturbance, and the conjecture may 

 be hazarded that it may have been caused by undermining resulting 

 from the erosion of lower beds of the Chalk along the face of the escarp- 

 ment. Other flexures in the Chalk will be noticed in the account of the 

 Eocene beds when treating of the outliers and inliers to which they 

 appear to have given rise. 



The Upper Chalk is a very permeable bed, and wherever it comes 

 to the surface it forms a dry porous subsoil. Only about 300 feet of 

 the lower portion of it are present in Hertfordshire. While the highest 

 beds were being deposited elsewhere, this part of England was probably 

 above the sea ; but the Chalk which has been deposited here has under- 

 gone an immense amount of waste, continuous from its final if not 

 from its first upheaval from the sea to the present time, and still going 

 on. There may, however, have been a time when the Chalk, or at least 

 the Upper Chalk, was entirely covered as it still is in the south-east of 

 the county, by the Tertiary beds, the clays of which would protect its sur- 

 face to some extent from disintegration. The great waste which it has 

 undergone is due, more perhaps than to actual denudation, to the gradual 

 dissolving of the carbonate of lime by water holding in solution carbonic 

 acid (or carbon dioxide) derived from the air or from decaying vegetable 

 matter. By this chemical action, which is continually going on, the 

 flints and insoluble clay in the chalk are left on its surface, and form 

 a deposit called ' clay-with-flints.' This covers a considerable area of 

 the Upper Chalk, chiefly in the western part of the county. By the 

 same chemical action also the so-called 'pipes' are formed, lines of 

 weakness in the Chalk allowing of the more rapid percolation of water 

 in certain places. Wherever it is not covered by an impermeable bed of 

 clay, these c pipes ' occur, and as their funnel-shaped mouths are some- 

 times of considerable extent, they give a very uneven surface to the 



1 Coleman, Flora Hertfordiensis, p. xxxi. (1849). 



* Geology of the North-west part of Essex and the North-east part of Herts, p. 8 (1878). 



IO 



