GEOLOGY it. It forms a rather steep escarpment to the north-west, from half a mile to two miles in breadth. At its summit is the Chalk Rock, a very hard bed of chalk, which varies from about i foot to 4 feet in thickness, and is sometimes, like the Totternhoe Stone, divided into two beds with a foot or more of the softer chalk between them. It is cream-coloured, much jointed, and has layers of green-coated nodules of equally hard chalk at the top, which is somewhat irregular, as if it had been exposed or subject to slight denudation before the next layer of chalk was deposited. Owing to its hardness, it has so far resisted denudation as to be usually found at or near the top of the Chalk hills which form the water-part- ing between the catchment-basins of the Ouse and Lea on the north and those of the Thame and Colne on the north-west. Owing also to its hardness it is a very difficult rock to work for fossils, but it is one which better repays the labour than any other in Hertfordshire. It forms the top of the Chalk escarpment south of Royston and north of Kensworth, and nearly all the tributaries of the Colne and Lea cut through it. There is a good exposure close to Markyate Street on the banks of a lane cut into it, and there was one in a chalk-pit south-east of Airley Green near Caddington ; but this has now been covered in. The best exposure, however, is in the Midland Railway cutting at Chiltern Green, but that is just outside our county, in Beds. Collections of fossils from these localities may be seen in the Hertfordshire County Museum at St. Albans. The Chalk Rock was first described by Mr. Whitaker, 1 partly from a collection of fossils made by Sir John Evans in the Boxmoor chalk-pit. It has recently been more fully described by Dr. Morison, and a list of its fossils found in the Chiltern Green cutting has been given by him. 2 Its Mollusca have been described, and many species have been figured by Mr. Henry Woods. 3 At the junction of the Middle and Upper Chalk are the highest hills of Hertfordshire, forming part of the north-easterly prolongation of the Chiltern Hills, and attaining an elevation, along the Royston, Luton, and Dunstable Downs, of from 400 to 600 feet generally ; and at Kensworth Hill, the highest point on the Dunstable Downs, of 8 1 o feet, being the greatest elevation in the county. The Upper Chalk, or chalk-with-flints, occupies much the largest area in Hertfordshire (at least three-fourths of the county). The general direction of the rivers of Hertfordshire is from north-west to south-east, and this corresponds with the slope of that portion of the county which is on the Upper Chalk. This generally forms an inclined plane, sloping downwards, with an inclination roughly coinciding with the dip or line of bedding of the Chalk, from the Chiltern Hills on the north-west to the valley of the Colne on the south, and that part of the valley of the 1 'On the Chalk Rock,' Quart. Journ. Geol. Sac., vol. xvii. p. 166 (1861). 2 'Notes on the Chalk Rock,' Trans. Herts Nat. Hist. Soc., vol. v. pp. 192-202 (1889). 3 ' The Mollusca of the Chalk Rock,' Quart. Journ. Geol. Sac., vol. lii. pp. 68-98 (1896) ; vol. liii. pp. 377-404 (1897). 9
