4 
54 George Clinch—Sculpturings of the Chalk Downs. 
‘* That the phenomena of the arrangement of valleys, and of watersheds, over all 
the length and breadth of the anticlinal line of the London and Hampshire basins, 
respond to this covvulsion. 
“¢ That this convulsion was attended or immediately followed by a devastating flood, 
which excavated and carried off the broken materials, and only left a small quantity 
of drift to attest its agency ; and that this inundation subsiding, the waters withdrew 
at once, a period of tranquillity succeeding, which has continued up to the 
present time.’’ 
Mr. Clement Reid’s paper,! ‘‘On the Origin of Dry Chalk Valleys 
and of Coombe Rock,” was published twenty-two years ago, and 
I think all geologists will admit the value of his contribution to the 
subject of Chalk hill-sculpture. 
Mr. Reid commenced his paper by pointing out the intimate relation 
of Dry Chalk Valleys and Coombe Rock, a position which I suppose 
no one would now wish to challenge. After a description of the sea- 
ward side of the South Downs, and of the general features of Dry 
Chalk Valleys, the author proceeded to discuss the various theories 
advanced by geologists to account for the erosion of the Dry Valleys. 
He cited three theories, viz. :— 
(1) Former submergence, and consequent rise in the plane of saturation. 
(2) A former higher level in the plane of saturation before the valleys had been 
cut to their present depth. 
(8) An enormous increase in the rainfall. 
All these theories were dismissed as insufficient to account for the 
phenomena of the Dry Valleys, although their partial aid in producing 
them was accepted. Mr. Reid remarks: ‘If these valleys had been 
gradually cut back by streams, many of them ought to fall northward 
to the escarpment, where most of the large springs are found; but 
nearly all the Chalk Coombes follow the general slope of the ground 
and open to the south.” 
The most important part of Mr. Reid’s paper is that in which he 
deals with the question of the origin and method of erosion of the 
valleys. Judging from the flora and fauna, he assumes that the 
temperature of North-West Europe, at the period under review, was 
probably 30° lower than it is now. He writes :— 
‘«This would give a mean temperature in the South of England very considerably 
below the freezing-point; consequently all rocks not protected by snow would be 
permanently frozen to a depth of several hundred feet. ‘This would modify the entire 
system of drainage of the country in a way that I do not think has been realized. 
All rocks would be equally and entirely impervious to water, and all springs would 
fail. Whilst these conditions lasted any rain falling in the summer would be unable 
to penetrate more than a few inches. Instead of sinking into the Chalk, or other 
pervious rock, and being slowly given out in springs, the whole rainfall would 
immediately run off any steep slopes like those of the Downs, and form violent and 
transitory mountain-torrents. ‘These would tear up a layer of rubble previously 
loosened by the frost and unprotected by vegetation. The material carried away 
would not have the Chalk washed or dissolved out, for a single flood of this description 
could have little solvent power, and much of the Chalk might not be thoroughly 
thawed. 
«« Each of these floods would have an enormous scouring and transporting power ; 
for the fall in the valleys is very great. It is noticeable that no Coombe Rock is 
found in the valleys that have a greater slope than 100 feet to the mile, and that the - 
main mass is deposited south of the Downs where the slope is much less.”? 
1 Q.J.G.8., vol. xiii, pp. 364-758 
