On the Geological-Survey map (Sheet 47, old series) a ‘line of flexure’ is marked in the Chalk-area from Therfield, south-west of Royston in Hertfordshire, to near Heydon in Cambridgeshire, a distance of rather more than 6 miles. The line is a curved one, and in three places to the north of it, and in one to the south, arrows are engraved on the map to indicate that the Chalk dips northward or north-north-westward at angles varying from 22° to 60°. It may be noted that the line of the flexure is drawn, not at right angles to the directions indicated by the arrows, but a little below the crest of the Upper Chalk-escarpment, which has here been eroded in a broad semicircular hollow; and that it is continued east of Heydon as the ‘approximate line of division between the Upper and the Middle Chalk.’ The late W. H. Penning, who surveyed the area, called attention to the flexure in 1876, when describing some ancient ‘river-gravels’ which lie in the vale to the north, but he did so only incidentally, remarking on its post-Eocene age, and expressing his opinion that the upper extremity of the Wardington valley—the semicircular hollow before mentioned—was due to this flexure in the Chalk. Particulars of the sections, with illustrations of those near Reed and Barkway, were published two years later in the Survey Memoir. Therein Penning (p. 11) stated that‘The flexure appears to have been very local, and of no great vertical extent. It occurs so