362 ME. H. B. WOODWAKB ON DISTURBANCES [Aug. I9O3, 



28. On some Disturbances in the Chalk near Royston (Hekt- 

 foedshire). By Horace Bolinobroke Woodward, Esq., 

 F.R.S., F.G.S. (Bead May 13th, 1903.) 



On the Geological-Survey map (Sheet 47, old series) a * line of 

 flexure ' is marked in the Chalk-area from Therfield, 1 south-west 

 of Royston in Hertfordshire, to near Heydon in Cambridgeshire, 2 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, 3 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 Bark way, were published two years later 

 in the Survey Memoir. 4 Therein Penning (p. 11) stated that 



' The flexure appears to have been very local, and of no great vertical extent. 

 It occurs so near the top of the escarpment that all evidence of the lower part 

 of the curve seems to have been obliterated by the denudation of the range. 

 Still, as there are no signs of Upper Chalk, but Chalk without flints occurs in 

 all exposures at a lower level, it is probable that just below the level of the pits 

 the beds begin to resume their horizontality.' 



With a dip of 60° there is here some demand on the imagination. 



The general structure of the area, although not free from 

 small faults, is otherwise quite normal ; and Penning rightly 

 observes {Joe. cit.) that 



' As a rule the Chalk lies very evenly, being either horizontal or dipping 

 slightly to the south-east.' 



The sequence, in fact, is regular from the low-lying tract of Chalk- 

 Marl, worked so extensively for cement-making at Shepreth, 

 through the Totternhoe Stone, the outcrop of which is marked here 

 and there by copious springs, and through the overlying grey blocky 

 Chalk or 4 clunch ' to the Melbourn Rock of Melbourn. Then comes 



1 Therfield, on the 6-inch map ; Tharfield, on the old 1-inch map. 



2 Until recently in Essex. 



3 Quart. Journ. Geol. Soc. vol. xxxii (1876) pp. 196, 197, 200. 



4 ' The Geology of the North- West Part of Essex & the North-East Part 

 of Herts, &c.' Mem. Geol. Surv. (1878) pp. 7-11. 



