part 1] SAXDS AXD GRAVELS AT LITTLE HEATH. 33 



typical Heading pebbles. Where the nocling has been lost, the 

 surface of the stone is often so smooth as to suggest ice-polishing. 



A very marked feature of this deposit is that the pebbles are 

 almost always in a vertical position, or highly inclined, even when 

 the laminae of the underlying beds remain horizontal. Occasionally, 

 the flints are found fractured along the vertical line, the two parts 

 remaining in close contact. In other places, the pebbles are crushed 

 in situ. This appears to suggest that the beds may have been 

 subject to ice-pressure either alter or in process of deposition. 



The clay matrix is tough and nighty mottled, and varies in tint 

 from very pale grey — almost white — to deep red or reddish purple 

 and brown, the deep colouring being on the whole the more per- 

 sistent. In the upper portion of the beds the colour is bleached 

 out, and this is also the case where rootlets have penetrated into 

 the clay. In the deeper sections, the dark colouring is more largely 

 retained. At the actual margins of the pebbles the mottling is 

 often very pronounced. It appears probable that the mottled 

 appearance is due to bleaching rather than that there has been an 

 admixture of clays from different sources and of different colours. 

 The tints of the highly-coloured clay leave little room for doubt 

 that it has been derived from the upper part of the Heading Beds. 



There is no sharp line of demarcation between these Glacial 

 beds and the underlying loamy sands ; in man}' cases the upper 

 surface of the sands has been cut into, and the beds are intermingled. 

 Indeed, the loamy sands for a foot or so below the junction are 

 usually found to be more or less disturbed. 



A singular fact in connexion with these beds is the absence of 

 the Chalk flints which are scattered in such profusion over all parts 

 of the surrounding country, and also the very rare occurrence of the 

 small pebbles of white quartz and lydite, so abundant in the under- 

 lying deposits. This appears to suggest that the ice must have 

 derived its materials from an area where these are very rare, and 

 the bleached Heading pebbles abundant. At present, such districts 

 have not been properly and exactly defined, the literature on the 

 distribution of the little quartz-pebbles being very incomplete. 



The deposit is extremely persistent over various parts of the 

 Little Heath and Berkhamsted Commons, overlying and often 

 cutting into and disturbing the beds beneath. On the south-west 

 side of the pit, for instance, is a pocket of the Glacial beds, about 

 12 feet deep, bending down the underlying loamy sands at a high 

 angle at the sides, and at the bottom cutting them out as well 

 as part of the gravel underneath. On the east side another pocket 

 is let in by a superficial fault or landslide, caused by the slipping 

 of the beds into a solution-hollow in the Chalk to about the same 

 depth (see PI. II). Mr. Whitaker in 'The Geology of London' 

 Mem. Geol. Surv. vol. i (1889) p. 83, refers to these as 'pipes ' in 

 the Chalk. In this district they look like large ponds when the 

 gravel or clay has been removed. They are sometimes several 

 hundred feet across, and comparatively shallow, and it is suggested 

 that the term ' solution -hollows ' better describes the phenomena. 



Q. J. G. S. No. 297. d 



