Knowledge. 



With which is incorporated Hardwicke's Science Gossip, and the Illustrated Scientific News. 



A Monthly Record of Science. 



Conducted bv Wilfred Mark Webb, F.L.S., and E. S. Grew, M.A. 



AUGUST, 1913. 



THE STREAMLESS DOWNS AND THEIR DRY 



VALLEYS. 



By G. W. BULMAN, M.A., B.Sc. 



One of the most striking things on the Chalk Downs 

 of the south of England is the absence of streams. 



" We have no waters to delight 

 Our broad and brookless vales, 

 Only the dew-pond on the height 

 Unfed that never fails." 



And the reason is obvious. The substratum of 

 chalk is so pervious that the rain sinks into the rock 

 at once, and none can run off to form streams. 

 Another feature of the Downs are the numerous dry 

 valleys, which look as if they had been carved by 

 water. We may call the coexistence of the two 

 features the puzzle of the physical geography of the 

 Downs. If there are no waters to cut them out, to 

 what do we owe " Our broad and brookless vales " ? 



In the " Memoirs of the Geological Survey " 

 (" The Geology of the Country round Eastbourne ") 

 we find the following suggestion by Mr. Clement 

 Reid :— 



" Such a feature in the pervious chalk cannot be 

 accounted for by any change in the amount of 

 rainfall ; it points to other conditions which have 

 now passed away. It is in all probability a relic of 

 the Glacial Epoch, which in these southern districts 

 did not lead to an accumulation of ice, but caused 

 the rocks to freeze to a great depth, thus rendering 

 them impervious to any rain that might fall in the 

 summer. During that period the chalk would be 

 cut into valleys in the same way as any impervious 

 rock, instead of immediately absorbing the heaviest 

 rain, as it does at the present day." 



Any suggestions coming from so distinguished a 

 geologist as Mr. Clement Reid must be received 

 with due respect, but there seems to be more than 

 one fatal objection to this explanation. It assumes, 



in the first place, that the porous chalk became 

 saturated with water to the surface, and was then 

 frozen hard. But if the rain sank into the chalk as 

 it does to-day this could never happen. Even after 

 the wettest season the line of saturation never rises 

 to near the surface. 



In the second place, is there any ground for the 

 belief that during glaciation the ground would be 

 frozen to great depths ? As a matter of fact, in 

 glaciated regions to-day ice and snow seem to keep 

 the earth warm. For there always seems to be 

 water under the glacier and ice-sheet. Mr. Reid, 

 however, seems to suggest that the southern parts 

 of the country were not covered with ice or snow 

 during glaciation. But we cannot hypothesise a 

 sufficiently heavy snowfall in the north to produce 

 glaciation, and none or very little in the south. Nor 

 can we suppose there was a summer rainfall 

 sufficient to cut the valleys without admitting a 

 corresponding snowfall in the winter. Would not, 

 then, the Downs be covered with snow and the 

 ground thus kept from freezing ? And even if 

 summer began with a frozen ground, would it not 

 begin to melt simultaneously with the first rain ? 

 We further venture to think that if these dry 

 valleys in the chalk had been carved during the 

 glacial period they would now show signs of filling 

 up. For there must be some movement of material 

 into them from the higher ground at the sides, and 

 there has been time for some to be almost obliterated. 

 But instead of thus growing less they seem rather 

 to be still in course of formation. 



We venture, therefore, to suggest as a tentative 

 hypothesis that our dry chalk valleys are the lines of 

 underground waters comparable to the Oueds of the 

 African deserts ; not as to origin, but in the fact 



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