cole: dry valleys in the chalk. 
345 
The discussion which, followed in the main approved of the 
author's views. 
The sight which I witnessed in January, coupled with ^[r. 
Reid's paper, leaves no doubt on my mind that melting snow, 
traversing a frozen subsoil of a few feet, might produce the running 
streams which so many are in search of, to account for the excavation 
of the Chalk valleys. This is a very different thing from saying, — 
1 , That at a former level of saturation, springs gushed out at the 
head of the dales, and formed surface streams ; or 2, That the dales, 
in lack of surface streams, were excavated by streams subterranean,. 
against both of which hypotheses I have always protested. At the 
same time we must not lose sight of the fact that similar dry valleys 
exist in parts of the world where we can hardly expect to find the 
friendly frozen subsoil, where, in fact, it never could have existed, 
and that these also have to be accounted for. 
"Whilst admitting, therefore, that glacial conditions were a vera 
c lavi in promoting the denudation of the Wolds, as already stated in 
my paper read before the society in 1879,* I still adhere to the 
opinion expressed in 1885,t that the main agent is the rainfall, 
acting not so much mechanically as chemically. 
All limestones are being slowly dissolved by the carbonic acid 
brought in contact with them by the rainfall, and their surfaces 
gradually lowered This can be proved by the fact that where a 
large boulder, or a tumulus of clay, acts as a covering from the 
rainfall, there the rock underneath is less wasted away than the 
surrounding parts. A trough-jhaped dale offers more surface to the 
rain than a level piece of ground of equal width, consequently the 
rate of chemical erosion, in addition to mechanical, is greater in the 
one case than in the other. In other words, the dales are being 
gradually widened and deapened at the expense of the table-land, 
the sides preserving the angle of repose, about 30.^ 
I endeavoiirei to show, by a series of tables^ in 1879, that the 
chalk area is being chemically lowered at the rate of 1 ft. in rather 
* Proceedings Yorkshire Geological Society. Vol. VII. Part II.: pp. 128 — 
140. Origin and formation of Wold Dales. 
t Proceedings. Vol. IX. Part I. : p. 114. Physical Geography of East 
Riding. 
X Proceedings. Vol. VII. Part II : pp. 139-UO. 
