122 PEOF. J. PRESTWICn ON THE RELATION OF THE 



oue of which he considered to be of Glacial a«-e, and the other 

 Post-Glacial ; and in the following year he noticed a small deposit of 

 pebble-gravel capping the hills near Hertford *, which he thought 

 might " be of Pre-Glacial age, but all that is known of it with 

 absolute certainty is that it is older than the Boulder Clay (which 

 occurs over it in other parts), and Mr. S. Y. Wood, Jun., has classed 

 it with his ' Middle Glacial.' " 



In 1875, and again in 1880, Mr. Whitaker noticed in more 

 detail t this " Pre-Glacial (?) Pebble-Gravel," and assigns to it more 

 definite limits. He says, " On the tops of the London-Clay hills 

 there is often a mass of sandy gravel of an exceptional sort, that 

 is to say, it is almost wholly wanting in the more or less angular 

 pieces of flint that form the greater part of the other gravels to be 

 described, and, like the far older gravel-beds of the Elackheath and 

 Bagshot Series, its component stones have been rounded into the 

 form of pebbles. Showing at first sight a very great likeness to 

 these old Tertiary pebble-beds, after a more careful examination 

 this gravel is seen to be easily distinguished from them ; for whilst 

 the former are made up of flint-pebbles, that is not the case with 

 the latter, which contains also a large proportion of pebbles of 

 quartz and quartzite, and here and there a sub-angular flint." 

 " The flint-pebbles have probably been derived from the destruction 

 of the old Tertiary pebble-beds ; but the pebbles of quartz, quart- 

 zite, and other older rocks that occasionally occur must have been 

 derived from beds that are not found anywhere in our district. 



" Of the age of this gravel we cannot yet speak with certainty. 

 It is newer than the Lower Bagshot Beds, for it is known to overlie 

 them : and it is older than the Boulder Clay, which is found above 

 it, but between these extremes we are left to reason by analogy and 

 by the evidence given by the manner of its occurrence." 



"Prom its occurrence on the tops of the hills, whilst the Middle 

 Glacial gravel often lies at their base, or on their flanks, it would 

 seem that the pebble-gravel is the older of the two and was 

 deposited long before those hills were cut into their present form, a 

 process which must have been somewhat advanced before the other 

 gravel was laid down. It is possible, therefore, that the pebble- 

 gravel may represent some part of the ' Lower Glacial Drift ' (of 

 Mr. Wood), any known occurrence of which is, however, as far 

 distant as the Crag." 



" The chief localities are Stanmore Heath from Shenley south- 

 eastward, west and north of Barnet, and at Totteridge, in Middle- 

 sex ; at Highbeech, Jacks Hill, and Gayne's Park, east of Epping, 

 in Essex, and at Shooter's Hill, in Kent." 



In the larger valuable memoir J, published since these pages were 

 written, Mr. Whitaker describes these Beds, which he places with 



* " The Geology of the N.W. part of Essex and the N.E. part of Herts," 

 Mem. Geol. Survey, Expl. of Sheet 47, 1878, p. 32. 



t " Guide to the Geology of London and the Neigh bourliood," Mem. GeoL 

 Survey (1st edit. pp. 49-51, and 3rd edit. pp. 54-57). 



+ • Geology of London,' vol. i. pp. 290-298 (1889). 



