VALLEYS AXD OE COOMBE BOCK. 



373 



thus frozen, the action of running water would be to excavate in the 

 manner claimed for it. He then proceeded to institute a com- 

 parison between the conditions obtaining in the North and South 

 Downs respectively, and stated that the absence of any deposit 

 representing the Coombe Rock in the north, and also the fact that 

 dry valleys existed throughout the Chalk area in Central France, 

 were difficulties in the way of accepting the Author's conclusions. 

 He gave some reasons for believing that the South Downs represent 

 a later stage of denudation than do the North Downs. 



Mr. Peach thought that the theory advanced by the Author 

 accounted for the phenomena of the dry Chalk valleys ; but in order 

 to make the Chalk impervious to surface-water, it was not necessary 

 to have it frozen to the depth of several hundred feet. The same 

 result would be brought about by the freezing of the ground for a 

 few feet below the surface. The Coombe Eock was evidently con- 

 nected with the excavation of these valleys. The presence in it of 

 numerous fragments of chalk and flints not rounded seemed to 

 indicate that its materials were laid down by streams flooded during 

 the summer seasons and much reduced in size or frozen during the 

 winters. The thawing of the surface during the summer would 

 set free angular fragments of chalk split by the previous winter's 

 frost, and the materials thus let loose would be swept down the 

 valleys over the still frozen substratum. In this respect the Coombe 

 Eock differs materially from the alluvial gravels of the existing 

 streams in that region, which contain abundant rounded flint 

 pebbles and few or no chalk fragments, the latter being disintegrated 

 or dissolved before they have travelled any distance. The decayed 

 character of the bones found in the Coombe Eock was in marked 

 contrast to the fresh state of the chalk fragments, and seemed to 

 indicate that they had been exposed to the weather prior to the 

 formation of that rock. 



Dr. Hlnde observed that in Canada, where the ground is solidly 

 frozen to a certain depth each winter, a greater amount of denuda- 

 tion is effected in 24 hours, at the sudden break-up of the frost in 

 spring, than takes place during the whole of the rest of the year. 



The Atjthoe, in reply, said that the valleys no doubt had been 

 largely outlined before the period dealt with in his paper, some of 

 them might be the continuation of old Weald valleys. The former 

 existence of an overlying clay deposit had something to do with the 

 original outlines. As evidence of the submergence spoken of by Mr. 

 Evans, he instanced the marine beach at Goodwood, 130 feet above 

 the sea. Such submergence occurred before the deposit of Coombe 

 Eock ; it might aid in the excavation of the lower valleys. As 

 regards rainfall, the fauna is Arctic, and there should be snow 

 rather than- rain, yet we have no evidence of true glacial action. 

 No attempt had been made at correlation with the deposits in the 

 north. In this district the old Tertiary deposits had gone, but it 

 was otherwise in the North Downs. 



