46 LONDON DISTRICT. 



Westwards in our district it has not been met with, but it 

 occurs at Bricket Wood, north of Watford. 



A small outlying mass of Boulder Clay, much weathered at 

 the surface, and about 6 ft. thick, was exposed (1908) at Oakleigh 

 Park, Whetstone; and beneath the clay a thickness of 8 ft. of 

 pebbly flint gravel was exposed; the junction between the two 

 was fairly even. 



To the south of Church End, Finchley. the Boulder Clay has 

 been exposed on both sides of the main road in recent excavations 

 for building sites, and in a trench west of the Manor House; 

 it occurs over parts of the St. Pancras, Islington, and Marylebone 

 Cemeteries, where sections may usually be seen ; a good account 

 of the exposures opened up when the railway-cutting at Finchley 

 Station was made has been published by Henry Walker. 1 



Much of the local material in the Boulder Clay was derived 

 from the subsoil that was temporarily frozen to the base of the 

 ice sheet, but material was transported from a distance at higher 

 levels in the ice, being shifted by thrusts and shear-planes in the 

 moving mass. In this way chalk detritus was scored by 

 fractured flint or different broken rocks, and other stones were 

 glaciated. 



Blocks of igneous rock from Scandinavia were probably 

 derived, in transit, from earlier accumulations of Boulder Clay, 

 belonging to the Lower Glacial Drift of East Anglia. Mr. G. 

 Potter has found rhomb-porphyry at Fortis Green, west of 

 MusweU Hill. 2 



That during the formation of the Boulder Clay the land 

 stood at a higher level than now, in reference to the sea, cannot 

 be doubted, when we consider the deep channels of erosion met 

 with in the Eastern Counties, in the Cam Valley, in Essex, at 

 Glemsford in Suffolk, as described by Mr. Whitaker, and at 

 Hitchin, as described by Mr. William Hill. At Glemsford the 

 drift extended to more than 350 ft. below O.D. 



It may be noted, however, that in ' Nature ' (7th July 1904, 

 p. 218) Mr. G. W. Lamplugh called attention to observations in 

 Alaska by G. K. Gilbert, who concludes that glacial ice may 

 continue to exert influence on the rock bed if it has descended 

 from the land into water and is not buoyed up, so ' that the depth 

 to which glacial troughs have been excavated is not demonstrative 

 of a relatively low base -level at the time of their excavation.' 



The discovery of a mass of Boulder Clay, 15 ft. thick, beneath 

 the 100-ft. terrace-gravel in the Thames Valley at Hornchurch 

 has been of great importance in showing that the fossiliferous 

 gravels and brickearth of the Thames Valley are newer than this 

 great glaciation. The Boulder Clay here occupied a hollow in 

 the London Clay, and was overspread by the Valley Gravel, but 

 the reading of this section has recently been disputed (p. 52). 



1 Proc. Oeol Assoc,, vol. ii, 1872, pp. 289-298. 



2 Quart. Journ. Oeol. Soc., vol. Ixv, 1909, p. 263; see also Woodward, H. B. : 

 and J. G. Goodchild, Proc. Oeol. Assoc., vol. x, 1887, p. 145. 



