26 THE KEY. EDWIIST HILL OIS' THE [Feb. I9I2, 



Boulder Clay in brickyards at AVoolpit, about 8 miles east of Bury 

 St. Edmunds (also at about 200 feet O.D.), and in the Culford 

 Brickyards, about 4 miles north-north-west of Bury (here at only 

 about 100 feet O.D.). Sands under Boulder Clay have been proved 

 at various levels (generally under 200 feet O.D.) in various well- 

 sinkings, as, for instance, Brettenham Park and Felsham.^ 



Along an extensive visible transition-line the action which 

 produced the clay has left the silts perfectly undisturbed : it 

 exerted on them neither thrust nor drag. In the lower of the two 

 Gallows-Hill Pits (3) and in the inner Alexandra Pit (6), at its 

 south-western end, the silts are seen to bend down into hollows ^ 

 10 or 20 feet deep, occupied by the clay. The beds are neither 

 thickened nor thinned, they lap over as sheets. I am inclined to 

 look on them as deposited on the sides of pre-existing hollows ; 

 but the hollows may be due to subsequent subsidence below. Some 

 tiny step-faults may be explained on either view, and I have found 

 no evidence which is decisive. 



IV. Gravels and Isolated Clay. 



The Stour Valley seems to be pre-Glacial. Boulder Clay was 

 found in its bed in a boring at Glemsford, about 4 miles above 

 Sudbury.^ The boring appears to show that the ancient valley- 

 bottom there lies 470 feet below the present floor. It may be 

 conjectured that this indicates a buried channel, and that such 

 channel lies buried all along the present valley. If so, the old 

 valley about Sudbury was deep and, in places, remarkably narrow. 

 However, all known sinkings have reached Chalk at moderate 

 depths. 



Whatever may have been the shape of the old valley, a great 

 mass of Boulder Clay must have been removed after deposition. 

 Thus we find the thickness of over 100 feet on the plateau reduced 

 to a few feet about the 200-foot contour : and at lower levels find 

 it sometimes absent altogether, as at the Ballingdon Chalk-pit (11), 

 the Ballingdon Victoria Brickworks (14), and the Little Cornard 

 Brickworks (15), each at about 100 O.D. 



The bedded sands and silts described in § II (p. 25) are 

 absent from all pits below the 200-foot contour. Those pits that 

 are called in the Geological Survey Memoirs ' Victoria Pits east of 

 the town ' (2) lie at a level below that of the Gallows-Hill (3) and 

 inner Alexandra (6) Pits. They show Chalk, presenting a hori- 

 zontal upper surface ; over it Thanet Sands, somewhat eroded at 

 the top ; upon these again Crag, with an extremely-eroded upper 

 surface ; and, finally, on this coarse gravels and sands, violently 

 current-bedded. Though only a quarter of a mile from the higher- 

 level pits mentioned above, with their 40 feet of silts, no silt is 

 here seen. In the coarse gravels lies a sheet, about 10 feet thick 



1 ' The Water-Supply of Suffolk ' Mem. Geol. Surv. 1906, pp. 32, 52. 



2 Mem. Geol. Surv. 1878, p. 53 : ' At the southern end. . .' 

 s Ibid. 1906, p. 58. 



