part 1] GAIILT AND LOWER GREENSAISTD NEAR LEiaHTON. 35 



The total thickness of the beds above the Sands at this place, 

 allowing for the dip, may be 17 or 18 feet. The clay-pits are 

 practically on the crest of a low rise, the ground falling slightly 

 to the south and east, and rather sharply to the west and north 

 towards the Ouzel and a tributary valley. There is no higher 

 ground near, therefore we may assume that the top clay, although 

 weathered and disarranged, is probably not far from its original 

 position. The presence of the keeled ammonites and of the 

 associated sulcated Inoceramus indicates the incoming of the 

 Upper Grault at the top of the section, and leaves a thickness for 

 the Lower Grault of approximately the same order as at Shenley 

 Hill (fig. 3, p. 7) and Heath House (p. 29). 



Several big sand-pits are being, or have been, worked west of 

 Orovebury, on the slopes of the Ouzel valley, and two of these are 

 near the top of the Sands ; but none has yet reached Gault in 

 place, and the sections, therefore, will not here be particularized. 

 They have revealed good exposures of gravelly drift and contorted 

 * trail,' with Boulder Clay in places, ^ directly overlying the Sands. 

 The drifts deserve further study, and I hope to continue the 

 investigation of them. 



In the opposite or eastward direction the main Grrovebury 

 sections terminate at the high road running south-eastwards from. 

 Leighton ; but I found in 1912 that new ground had been broken 

 on the farther or east side of the road, immediately north of the 

 railway at ' Billington Crossing.' The section, known as Pratt's 

 pit, was similar to that of the Spinney pit (p. 33), but afforded a 

 better opportunity for obtaining fossils from the gritty phosphatic 

 nodules. The pit was visited in 1915 by members of the Greologists' 

 Association under my guidance, and the first example of ' Ammo- 

 nites mammillatus ' to be obtained from the bed was found on this 

 occasion. The section at the eastern end of the pit, where the 

 Oault was thickest, was figured and described in my report of 

 the excursion (^op. cit. p. 311). The text of the description is 

 reproduced below (repetition of the figure being unnecessary). 

 This part of the pit has since been worked out and filled in, and 

 the present workings have reached northwards beyond the feather- 

 edge of the Grault, but under an increasing cover of gravelly 

 drift. 



^Section at the eastern end of Pratt's pit, Billington Crossing, 

 Leighton Buzzard; 1912-1915. 



Thickness in feet. 



Z. Clayey soil, with a few stones 1 — 2 



Y. Pockets of drift-gravel, consisting mainly of flint and quartzite- 



pebbles 0— 0|^ 



2^- Tough dark-blue unfossiliferous Gault clay, weatliering brownish 

 4 and with streaks of ' race ' in the upper part ; becoming gritty 



and including a few phosphatic nodules in the lower part (4a) : 2 — 3 

 passing down into — 



^ For some notes on one of these sections, see Proc. Geol. Assoc, vol. xxvi 

 ■(1915) p. 312. The position of another is shown in the sketch-map (fig. 1, 

 p. 2) which falls, however, short of the other sites. 



d2 



