part 1] GAULT AISTD LOWER GREElS^SAlSrD NEAR LEIGHTON. 57 



observations, particularly by the new facts disclosed in Grarside's 

 pit. as already described (pp. 10-13) and the similar features 

 displayed in the Nine Acre and Miletree pits (pp. 16-19). It is 

 significant that this massive induration should occur mainly in the 

 neighbourhood of the bomidarj^ of the Silty beds ; and it is at least 

 probable that the mineralizing solutions may have been derived in 

 the first instance from the mixed and unstable materials of these 

 beds. In shallow-water deposits of this type, if a slight elevation 

 €nsued after their deposition, so that they were brought under the 

 influence of weathering, the waters percolating from the surface 

 would become charged, as they still often are, with mineral matter 

 in solution, wdiich might be redeposited in the cleaner beds below. 



But whatever may have been the cause of the induration, we can 

 see from the sections that it had become effective at an early date; 

 so that when, by another change of conditions, there was a sharp 

 renewal of current-action on the shoal, and possibly wave-action 

 also, during the stages ushering in the great marine transgression 

 of Upper Cretaceous times, the old sand-banks yielded very 

 unequally to the attack. The shoal, protected by its irregular 

 ■capping of iron-grit, was scoured into crags and rocky ledges, which 

 were undermined along their edges, and broken down into a breccia, 

 but continued to persist in patches as a dissected reef, until, with 

 the deepening of the water, the severity of the attack died down 

 and passed gradually into the tranquil conditions required for the 

 deposition of the Gault clays. In the sketch-map (fig. 1, p. 2) 

 the cross-road from Heath past Shenley House to Clipstone runs 

 nearly along the southern margin of the old reef. 



It has already been pointed out how closely this sequence of 

 •events agrees with that occurring during the same period in North- 

 Eastern France, where the floors of Paleozoic and Mesozoic strata 

 underlying the Upper Cretaceous rocks have been similarly scoured 

 and guttered. 1 The conditions are also repeated, on a modified 

 scale, at the base of the Ked Chalk along the western edge of the 

 Yorkshire Wolds (see p. 64). 



The Basement beds (3). — The difficulties and misunder- 

 standing which have arisen in respect of the correlation and classi- 

 fication of the peculiar and variable beds immediately underlying 

 the Gault clays in the sections described, have been caused mainly 

 by the difference between the aspect of these beds in the Grovebury 

 sections and in the Shenley and other sections about 2 miles farther 

 north. This difference, however, is now bridged by the association 

 of the two separate types of deposit in the intermediate section 

 at Chamberlain Barn pit ; and my original surmise as to the 

 relationship of the tj^pes ^ is justified. The difference is mainly 

 due to the persistence, for some time, of shoal-water and reef- 



' See footnote and reference, ante, p. 18 : also C. Barrois, ' Terrain Cretac^ 

 des Ardennes, &c.' Ann. See. Geol. Nord, vol. v (1878) pp. 278-80. 

 2 Proc. Geol. Assoc, vol. xxvi (1915) p. 310. 



