The Evolution of the Relief of the Cam Valley

Abstract

East Anglia and adjoining districts, with their wealth of glacial deposits and monoto ous relief, have stimulated re earch in o p obl ms of drift geology, but the development of relief has been largely neglected. The valley of the river Cam is no exception, for the papers concerned with its drift geology make little or no mention of the problems of relief. The Cam is one of the smaller rivers draining the region surrounding the Fens: it has three branches which, because of confusion of names, will be called the Ashwell, Saffron Walden and Linton branches, after the main settlements found in the valleys. The river system is short: from its farthest source near Elsenham to its junction with the Old West River near Stretham it is little more than 30 miles long. The area drained by this system falls into three natural divisions (Fig. 1). To the south-east is a low Chalk plateau with a variable but usually considerable thickness of boulder clay. The plateau is bounded to the north-west by a gentle and irregular escarpment which, from its indented character and its broad, shallow coombs, appears to be of considerable age and to have suffered little erosion for a very long time. Nowhere does it approach in steepness the characteristic Chalk escarpments of the south of England. The crest of the escarpment is highest in the south-west, where it reaches 550 feet O.D. south of Royston. It falls in elevation north-eastwards to 380-400 feet O.D. near the Saffron Walden valley and retains that elevation as far as Newmarket. Even where it is highest it is not impressive, for an area of lower dissected plateau at about 300 feet O.D. lies in front of it. The general pattern of valleys on the plateau follows the slight dip to the south-east, but the Linton and Saffron Walden streams are exceptional in flowing against the dip. The latter is especially noteworthy, for it rises some 10 miles south of the Chalk escarp? ment and is an obsequent stream of unusual length for Chalk regions. North of the plateau lies the broad strike vale of the Ashwell stream, a valley of unusual size in relation to the stream occupying it. If the zone of terraces on the southern side of the valley is included, it is slightly more than 6 miles wide near its source at Ashwell and retains this surprising width until it merges into the Fens north of Cambridge. The zone of terraces on the southern side of the valley is cut mainly into the Middle Chalk and the Lower Chalk above the Totternhoe Stone, while the lower, more modern valley is cut indifferently across the Chalk Marl and the Gault (Fig. 1). These two rocks are almost equally lacking in resistance to erosion and, from a geomorphological point of view, the important lithological break is not the base of the Chalk but the level of the Totternhoe Stone. North-west of the Ashwell valley a low plateau extends between the Cam and the Ouse valleys. This western plateau is of monotonous relief, reaching 270 feet O.D. in the south centre and sloping gently down usually to 200 feet, but locally in the north to 160 feet O.D., above the break of slope to the adjacent valleys. The plateau is essentially a boulder clay feature, for solid rocks outcrop in very few places on its sur? face, although the Lower Greensand comes to the surface in the south-west near Sandy and the Chalk in the south-east near Barrington. The surface of the solid rocks, which include all those locally represented between the Oxford Clay and the Chalk, is much lower and more irregular than the surface of the drift forming the plateau. The plateau is dissected by the Bourn Brook, which flows eastwards into the Cam and separates the eastern part of the plateau into two main ridges, one extending to Madingley a few miles west of Cambridge and the other to Barrington.