104 



J. V. ELSDEN POST-TERTIAKY 



representing some part of the Lower Glacial drift ; but all that 

 is certainly known of them is that they are the oldest drift of 

 our district.* Beyond these gravels of doubtful age there are no 

 other deposits in our county of the Lower Glacial period. 



Fig. 1. — Gravels of the two Plains of Hertfordshire. 



RRtCKr/'DKII 



I 6re£n 



D. Boulder-clay. G. Gravel of the Lower Plain. H. Gravel of the Upper 

 Plain. C. Tertiaries and Chalk. 



Middle Glacial. — The beds of this age, called by Prof. Hughes 

 " Gravels of the Lower Plain," vary far more in composition 

 and arrangement than those previously described. They contain 

 very many sub-angular flints, looking broken and weathered, as if 

 derived from an exposed flinty soil. Occasionally flints occur 

 scarcely rolled or broken at all, as if derived directly from the 

 Chalk or Boulder-clay. There is often much false-bedded sand, 

 and about the middle of the deposit there is generally a bed of 

 brown loam and clay, passing sometimes into Boulder-clay, with 

 drifted Oolitic fossils, and rolled and ice-scratched lumps of chalk. f 

 Many of the valleys, running south and east from the chalk es- 

 carpment, cut through boulder-clay and expose beneath it these 

 Middle Glacial sands and gravels, which extend probably over 

 nearly all the county between the Chalk and Boulder-clay. But 

 in no instance do they run up to the top of the chalk escarpment 

 or occur on the north-west face of it.j While the Boulder-clay 

 extends right over the chalk escarpment, the Mid-Glacial gravels 

 end a few miles lower down. (See Pig. 2.) 



Pig. 2. — Section from the Valley of the Stort, through the Chalk 

 Escarpment, to the Valley of the Cam. 



Bjr.|roar 



D. Boulder-clay. E. Mid-glacial. E. London Clay. M. Beading Beds. 



C. Chalk. 



* ' Quart. Joiu'n. Geol. Soc.,' vol. xxiv, p. 464. 

 t Ih. p. 285. 



X 'Mem. Geol. Survey on Sheet 47,' p. 34, and 'Quart. Journ. Geol. Soc.,' 

 vol. xxxii, p. 191. 



