A HISTORY OF HERTFORDSHIRE feet, but its limits are not clearly defined in well-borings, and it is almost certain that too great a thickness has been assigned to it there. This may also be the case at Cheshunt. From the softness of the Chalk Marl the Lower Chalk at first forms a continuation of the Gault plain, and then a gentle upward slope along the denuded edge of its escarpment up to its junction with the Tottenhoe Stone. This is a hard, rather sandy chalk, from 6 to 1 2 feet thick, and often occurs in two beds, each 3 or 4 feet thick, and separated by a few feet of marly chalk. From its hardness the Totternhoe Stone stands well out above the plain of softer strata, usually forming a distinctive feature in the landscape. It enters the county in the Cam district, at Ruddry Spring near Ashwell, passes through the Ivel district north of Baldock, near Cadwell north of Hitchin, and by Pirton and Hexton. It then enters Bedfordshire, in which county it forms the ridge of Sharpenhoe Knoll, and it has long been extensively worked at Totternhoe as a building-stone, but has not been quarried there for some years. It decays rapidly when exposed to frost and other effects of the weather, as the present state of the west front of Dunstable Priory Church, which is built of it, testifies. It should not be employed for exteriors, but it is admirably adapted for interior decorative work, being at first soft and easily manipulated, and hardening and becoming whiter as the moisture dries out of it. The last which is seen of the Totternhoe Stone in Hertfordshire is in the Thame district north-west of Tring, where it crops out near the summit-level of the Grand Junction Canal south-east of the reservoirs, then forming the ridge of the hill which extends for some distance along the south-eastern side of the Wendover Canal. Nearly all along the outcrop of the Totternhoe Stone there are springs at frequent intervals which give origin to deep combes in the north-western escarpment of the Lower Chalk. The water in several instances soon disappears from the surface, being absorbed into the Chalk Marl ; the combes then being formed, or perhaps merely deepened, by underground denudation. The rest of the Lower Chalk consists of about 60 to 90 feet of hard grey and white chalk, followed by 4 or 5 feet of grey marly chalk. There are two other hard beds in the Chalk of Hertfordshire, the Melbourn Rock and the Chalk Rock. The most recently expressed view is that the former divides the Lower from the Middle Chalk, and the latter the Middle from the Upper Chalk ; but this gives so many divisions to the Chalk that it is best here to consider the Middle Chalk as having the Melbourn Rock at its base and the Chalk Rock at its summit. The Melbourn Rock is a hard, yellow and white, bedded nodular chalk, about 10 feet thick. It may be well seen in a small pit just below Willbury Hill, and it partly surrounds Ravensborough Castle, which is not really a castle but an ancient camp, five miles west of Hitchin. A bed of white chalk, which varies in thickness from about 200 to nearly 350 feet, follows. It is more silicious in composition than the Upper Chalk, but has only a few flints irregularly distributed through 8
