T 



[ 1024 J 

 CX. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 

 [Continued from p. 872.] 



May 25th, 1921.— Mr. E. D. Oldham, F.K.S., President, 

 in the Chair. 

 HE following communication was read : — 



On the Junction of Gault and Lower Greensand near Leighton 

 Buzzard (Bedfordshire).' By George William Lamplugh, F.R.S., 

 F.G.S. 



The paper, a continuation of one by the author and the late 

 J. F. Walker published hj the Society in 1903, describes about 

 twenty sections exhibiting the base of the Gault in sandpits and 

 other excavations around Leighton Buzzard and westward at Wing 

 and Long Crendon. 



The variable ' Basement Beds ' of the Gault are ' condensed ' 

 deposits, strongly influenced by local conditions like the ' Tourtias ' 

 of Flanders, and falling mainly within the ' zone of Ammonites 

 mammilla tus ' as recognized in Northern France (= zones of A. 

 regularis and A. tardefurcatus of a later German classification). 



The evidence bears out Jukes-Browne's suggestion of the 

 occurrence of a current- swept strait in this quarter during late 

 Lower Cretaceous times, uniting northern and southern sea-basins. 

 During the accumulation of the 'Basement Beds.' a shoal in this 

 strait north of Leighton formed a reef capped by ferruginous ' pan ' 

 and breccia, with lenticular patches of shell-limestone preserving a 

 fauna of ' reef-facies,' while the deeper water to the southward 

 gathered a stratum of gritty glauconitic loam and clay with 

 fossiliferous phosphatic nodules of the French ' coquins de sable ' 

 type. The transitional stages are visible in the sections. 



The dark clays above the ' Basement Beds ' belong to the Lower 

 Gault, here reduced to about half its thickness at Folkestone, the 

 same reduction being exhibited also toward the opposite edge of 

 the basin, in Northern France. These clays rest sharply on the 

 ironstone ' pans ' of the reef, but usually pass downward by gritty 

 intercalations into the glauconitic loams. Fossils other than 

 ' Belemnites minimus' and 'Inoeerarnus concentricus ' are scarce and 

 in poor state, but are in agreement with the stratigraphical evidence. 



The incoming of the Upper Gault, with keeled ammonites of 

 the ' rostratus ' group and ■ Inoeerarnus sulcatus,' is shown in three 

 of the sections, of which, however, two are at present obscure. A 

 band of corroded phosphatic nodules, like those of the ' Junction- 

 Bed ' at Folkestone, occurs near the base of the division, and marks a 

 long pause in the sedimentation. This band has yielded many fossils. 



The palaeontology of the deposits is discussed, and is held to be 

 in general agreement with that of the same succession in Northern 

 France. 



A recent suggestion that the beds at Shenley Hill may have 

 been inverted by Glacial agency is fully considered, and shown to 

 be untenable. 



