A HISTORY OF HERTFORDSHIRE Clutterbuck is our first historian who gives any precise information on the geology of the county. Under the heading of ' Natural History and Climate ' l he gives a brief description of the Totternhoe Stone and of the Hertfordshire Conglomerate. This appears to be the earliest notice of the latter except the following curious account in a work dated 1756, relating to the Natural History of the county, as quoted by Young Crawley, 2 who gives neither the title of the work nor the author's name. ' The surface of every ploughed field is covered with innumer- able small Stones of the flinty kind generally, and many of them inimit- ably variegated with various Colours and Figures. The Plumb Pudding stone may also be called a native of this County. Many of this kind which are found here will weigh twenty or thirty pounds, and will bear as fine a polish as Glass, and far exceed in beauty all the Marble I ever saw. In many of their gravel pits are also found clear, transparent peb- bles, generally not exceeding the size of chestnuts, and seldom less than a pea, but as clear as a drop of water, and extremely hard. These cut and polish as fine as a Diamond, and when set upon a good foil appear extremely brilliant, and are capable of being made into Rings, Buttons, and other Toys.' The geological formations represented in Hertfordshire, 3 with their chief lithological characters and approximate thickness, are given in the following table, in descending order, the names of the formations which do not come to the surface in the county being printed in italics. 4 1 History of Hertfordshire, vol. i. p. iii. (1815). 2 Guide to Hertfordshire, 'Introduction,' p. 9 (1880). 3 For a complete account of these formations reference should be made to the Memoirs of the Geological Survey, especially The Geology of London and of Part of the Thames Basin, by W. Whitaker, B.A., F.R.S., 2 vols. (1889). 1 The coloured section with the map shows the escarpment of the Chalk and the out- crop of the underlying Secondary rocks beyond Pitstone Hill, with the false escarpment of the Upper and part of the Middle Chalk at Moneybury Hill, the hollow between these two hills being a dry valley. The Chalk Rock at the summit of the Middle Chalk and the Melbourn Rock at its base are shown by double lines, and so is the Totternhoe Stone near the base of the Lower Chalk. Outliers of the Eocene Beds over the Chalk are illustrated by the one at St. Albans, and inliers of the Reading Beds underneath the London Clay by that at Cough's Oak. A slight anticline in the Chalk is indicated here. The dip of the Chalk from the swallow-holes at Potterells near North Mimms shows how water sinking in there will find its way along the interstices in the layers of flints into the valley of the Lea rather than into that of the Colne. The plain section in the text (p. 4) shows the position and dip of the Silurian and De- vonian rocks where proved to be present beneath an uneven under-surface of the Gault. It is evident that the Devonian rocks must rest unconformably upon the Silurian nearly 1,000 feet beneath the surface somewhere between Hertford and Turnford. The unconformity between the Secondary and the Palaeozoic rocks is seen to be rather greater than that between the Silurian and Devonian. The thinning-out of the Lower Greensand towards the south- south-west and the thinning-out of the Upper Greensand in a north-north-easterly direction are indicated. As in the other section, the Chalk Rock, Melbourn Rock, and Totternhoe Stone are represented by double lines. An Eocene outlier near Bennington has inadvertently been omitted to be shown. The horizontal line through the section (appearing by an optical illusion to dip to the left) indicates Ordnance datum. In each section the vertical scale is twelve times the horizontal, the latter being the same scale as that of the map, on which the trend of each section is indicated by a thin black line. 2
