Next comes a bed of about four feet thick known as the Chalk-rock. It is a hard cream-coloured rock, containing layers of green-coated nodules, is traversed by numerous vertical joints, and rings to the stroke of the hammer. Owing to its hardness, it resists the action of the weather, and is therefore in evidence at or near the summits of the hills, where it can be traced from close to Berkhampstead Castle by Boxmoor and Apsley, and thence to the south-west of Dunstable, Kensworth, the south of Baldock, and so in a north-easterly direction to Lannock Farm. [Illustration: View on the Downs looking towards Wallington from the Icknield Way] Below the Chalk-rock we come to the Middle Chalk, or Chalk without flints, which may be so much as 350 feet in thickness, and rises in a rather steep slope or “step” from the underlying beds to be next mentioned. Flints are few and far between in the Middle Chalk, which forms the western slope of the Downs at Royston, as well as beyond the limits of the county at Luton, and so on to the Chiltern Hills. Fossils are much more numerous in the Middle than in the Upper Chalk. The lowest bed of the former is the Melbourn Rock, a hard, nodular band about 10 feet thick. Next comes the grey and white Lower Chalk, from 65 to 90 feet thick, after which we reach the Totternhoe Stone. Although only six feet in thickness, this Totternhoe Stone, which forms the escarpment of Royston Downs, is of importance as having been largely employed in the construction of churches and other buildings on the northern side of the county. It is a sandy grey limestone, which used to be largely quarried at Totternhoe, with special precautions in drying. It can be traced from Tring by way of Miswell, Marsworth, Pirton, and Radwell to Ashwell. The Totternhoe Stone really forms the top of the Chalk-marl, which is some 80 feet thick, and consists of buff crumbling marly limestones. It forms a strip of low ground at the base of the Chalk escarpment. At the bottom of the Chalk occurs the so-called coprolite-bed, which contains large quantities of phosphate nodules. Forty years ago these beds were extensively worked between Hitchin and Cambridge for the sake of the nodules. Only on the northern border of the county, between Hitchin and Baldock, and also near Tring and then merely to a very small extent, are any of the beds underlying the Chalk exposed. These comprise, firstly the Upper Greensand, which is either a sandy marl or a sandstone with green grains, and secondly, a dark blue impervious clay known as the Gault. These formations constitute the plain at the foot of the Chalk hills in Bedfordshire, the scenery of which is very similar to that of the London Clay plain in eastern Hertfordshire and Middlesex.