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