H. A. Baker — Quartzite Pebbles of the Oldhaven Beds. 69 



Blackheath Pebble Beds exposed in a road-cutting on the eastern 

 side of Plumstead Common. In addition to showing all the usual 

 features of these quartzite-pebbles, the present specimen is specially 

 interesting in virtue of the fact that it contains within itself a large 

 number of rolled flint-pebbles. Numbers of the latter were exposed 

 on the exterior of the pebble, and on fracturing it many more were 

 seen. Slices were prepared for microscopic examination, and were 

 cut so as to include some of the flints. Two microphotographs of 

 these sections are shown (Figs. 2 and 3). The amount of cementing 

 material seen in these slices is less than was seen in sections of other 

 quartzite pebbles (as mentioned above, variation in the amount 

 of cement is characteristic of these rocks), and there is a closer 

 approach to a quartz-mosaic structure, but in other respects all the 

 usual features are exhibited. One point, however, is worthy of 

 mention. In addition to the flint-pebbles, rolled quartz-pebbles 

 were also present. These ranged in size up to about | in. in diameter. 

 Fortunately some were cut through in the slices, so that it was 

 possible to determine by microscopic examination that they were 

 quartz-pebbles and not altered flint. The point is of interest since 

 the recorded instances of the occurrence of quartz-pebbles in the 

 Eeading Beds are few. Mr. H. J. Osborne White has stated that 

 the occurrence of these pebbles at the base of the Beading Beds 

 is by no means rare, but they appear to have been recorded only 

 from Long Valley Wood, near Eickmansworth, Lane End, Nettlebed 

 Hill, and near Chalfont St. Giles.' 



The above evidence definitely fixes the age of this quartzite- 

 pebble (and presumably the others) as post-Cretaceous and not 

 younger than the Oldhaven (Blackheath) Beds. The pebble presents 

 the typical flattened ovoid shape and is particularly well rolled. 

 As is well known, the rounding of the pebbles in the Blackheath 

 Pebble Beds is of a very thorough character — in the typical exposures 

 a subangular flint is unknown. The character of the beds suggests 

 that their deposition occupied no great length of time — the time 

 required for the rolling of the pebbles was probably great in com- 

 parison. That is to say, the rolling of the pebbles probably 

 preceded their deposition as the Blackheath Pebble Beds. A 

 well-marked plane of erosion separates the Pebble Beds from the 

 underlying Woolwich and Eeading Beds, and sections showing 

 " scoops ", where the Pebble Beds are seen to rest on low members 

 of the Woolwich and Eeading Series or sometimes to cut completely 

 through them and rest on the Thanet Sand, are of common 

 occurrence. It would appear that during this pre-Oldhayen 

 erosion of the Woolwich and Eeading Beds, large numbers of flints 

 and occasional sarsen-stones were removed from them, subjected 

 to a great amount of rolling, under conditions which did not, as a 

 rule, permit of the decomposition of the pebbles, whereby the 

 rounding of the latter was perfected, and that finally the pebbles 

 1 Proc, Geo]. Assoc, vol. xxi, 1909-10, p. 245 (with further references). 



