part 1] GAULT AlS^D LOWER GREEIN^SAND NEAE LEIGHT0:N'. 31 



their grittiness appears to vary with the grittiness of their 

 enveloping matrix, those in the more clayey upper layers being o£ 

 smoother texture than those below. 'J.lie fossils are mostly in 

 the form of casts, and are only present in recognizable shape in a 

 small percentage of the nodules, though few are without some 

 indication, more or less obscure, of former organic structure. It 

 is possible that prolonged investigation might reveal some difference 

 in the contained fossils from different levels, but only a few nodules 

 are available at a time ; most of my specimens of ' Ammonites 

 beudanti' were found in the upper part of 3b, and of 'Ammonites 

 regularis ' in the middle part, but the numbers obtained are too 

 small to Avarrant a positive statement. 



The breccia is more heterogeneous than the corresponding band 

 in the Shenley Hill group of pits, and is less calcareous. Exclusive 

 of those of iron-grit, it contains more large pebbles ; among those 

 Avhich I collected are — one of veined felspathic grit (?), 3|x3x2 

 inches ; two of quartzite, respectively 2| X 1^ X linch and 1| x 1| 

 X 1 inch ; and another of hard flaggy sandstone of similar dimen- 

 sions. Besides these pebbles, I noticed several worn subangular 

 lumps of hard conglomerate ^ up to 6 or 8 inches in diameter, 

 composed of smooth pea- or bean-sized pebbles, mostly quartz, set 

 in a dense pale-buff or pinkish calcareous or phosphatic cement, 

 evidently derived from the breaking-up of a local cake or lenticle 

 of winnowed Lower G-reensand pebbles which had become bound 

 in a limy paste. A few smaller lumps of the same conglomerate 

 were also found in the clayey grits above the breccia. Some frag- 

 ments of the same rock occur in the breccia at the Miletree pit 

 (fig. 10, p. 19). 



In the present (1920) working-face of the pit south of the 

 tramway section, the rise of the Sands soon cuts out the overlying 

 beds, leaving only a few nodules and hard fragments in the top- 

 soil as a trace ; but northwards the section continues, as in fig. 16, 

 nearly to the end of the pit, at the fence adjacent to Leighton 

 Farm. On reaching a shallow depression of the ground near the 

 fence, the gritty clays are truncated by a wash of loamy drift 

 with flints, etc., which has accumulated to a depth of 6 or 8 feet 

 in a hollow, cut down into the Sands since Grlacial times. 



The G-rovebury pits. — Other pits showing the fossiliferous 

 Mammillatus beds, but without the ironstone-breccia, lie along the 

 southern or Grove bury outskirts of Leighton Buzzard, adjacent to 

 the Dunstable Branch of the London & North- Western Railwav. 

 These sections are a mile and a half south of Chamberlain Barn 

 pit and 2 miles south-south-east of the Shenley Hill workings 

 (see map, fig. 1, p. 2). Some small pits in the intervening tract, 

 now abandoned, have shown only the Leighton Sands and gravelly 



^ At a casual glance the conglomerate looks not unlike some varieties of 

 the ' Hertfordshire Puddingstone,' but is differentiated from it sharply by the 

 absence of flint-pebbles and by the character of the matrix. 



