232 Notes on the Gravels of Croydon and its Neighbourhood. 
distributed over the plateau are of fairly large size, and ap- 
parently but little rolled.: 
(C). Rounded fragments of yellow, porous chert rock, made 
up of sponge spicules and their hollow casts. There can be 
hardly any doubt that these have been derived primarily from 
the siliceous sponge-rocks of the Hythe division of the Lower 
Greensand, like those of the higher portions of the Lower Green- 
sand ridge between Sevenoaks and Hindhead. These cherty 
fragments are sparingly distributed over the high chalk plateau 
in connection with the clay-with-flints, and they are numerous 
in the older plateau gravels, as in those of the watershed near 
Walton-on-the-Hill, and from these sources they have passed 
into the valley eravels, where, however, they are but seldom met 
with. This chert rock, and probably the iron sand as well, has 
been brought from the outcrops of the Lower Greensand to the 
south and dispersed over the chalk plateau at a period anterior 
to the formation of the present valleys. 
I have not met with any fragments or pebbles of white quartz 
or quartzite (beyond those from the Blackheath beds), or of any 
older rocks, either on the plateau or in the valley gravels of the 
Wandle area, and this. negative feature markedly distinguishes 
them from the gravels of Wimbledon Common and Combe Wood, 
Kingston Hill. 
It has long been evident to geologists that climatal forces 
like those now prevailing are insufficient to have caused the 
erosion of the dry valleys of the chalk and the transport of the 
large masses of gravel with which the lower part of the valleys 
and the flat areas to the north of our district are now covered, 
and various explanations have been proposed to account for 
these phenomena. It has been supposed that at the time they 
were formed there was an excessive rainfall, some ten times the 
amount of that which now occurs, but.there is no evidence from 
other quarters to support this hypothesis, and it may be doubted 
whether even a rainfall of the extent imagined would, have 
sufficed to erode these valleys. 
A more probable explanation is that brought forward lis Mr. 
Clement Reid, F.G.8.,* who considers that during the Ice Age, 
when glaciers reached to the edge of the Thames valley on the 
north of London, the climate was sufficiently rigorous to have 
caused the surface beds of the chalk to be permanently frozen, 
and that in this condition the rains during the summer months, 
with the water from the melting snow, would erode the chalk 
surface as if it were a non-porous rock, and thus the valleys 
would be excavated, and the harder flints transported to the 
lower levels. : 
* *Quart. Journ. Geol. Soc.,’ vol. xliii., 1887, p. 364. 
