VALLEYS AND OP COOMBE EOCX. 



369 



Of the effects of the hypothetical -'Pluvial Period" I have 

 nowhere been able to find any trace in the Coombe Pock. One 

 would expect such a period to be characterized by a prolific aquatic 

 fauna and flora. But instead of this, nearly all the Pleistocene 

 freshwater deposits I have been able to examine show a remarka- 

 ble paucity of purely aquatic forms and a considerable development 

 of amphibious species, such as can survive for a long time buried 

 in mud. 



Again, a Pluvial Period would enormously swell the rivers, 

 which would cut deeper and wider channels, transport more gravel, 

 and wear the stones more. Though we find abundance of deep 

 channels .cut through the Downs, the most marked peculiarities of 

 the Coombe Pock are that it spreads mainly in broad nearly level 

 sheets, has been almost entirely swept out of the valleys, and is yet. 

 very little worn. 



The peat-deposits, such as a Pluvial Period ought also to have 

 left abundantly interbedded in its alluvia, are entirely absent from 

 the Coombe Bock. It has been a great disappointment to me to be 

 unable to obtain any plant-remains except a few pieces of decayed 

 pine-wood, possibly derived from an older deposit. 



From what has just been pointed out, it seems that none of the 

 theories usually accepted is sufficient to account both for the origin 

 of the Coombes and for the transport of the Coombe Eock. 



Coombes are not forming now — in fact r many are so steep and 

 narrow that they are gradually filling up from the chalk and flint 

 rubble which rolls down the slopes, dislodged by the sheep. Coombe 

 Rock also is not now being formed. The gravels of the Lavant and 

 other chalk-streams are of a totally different character, and the 

 rainwash and talus now accumulating at the foot of steep slopes are 

 also different. 



As no denuding agent now at work in the south of England 

 seems sufficiently energetic to account for the transport of this mass 

 of rubble, and similar deposits are not well represented in glaciated 

 districts, it was at first very difficult to explain its origin. 



However, we know from the evidence of fossils that during some 

 part of the Pleistocene Period a thoroughly Arctic fauna and flora 

 lived in the south of England. At Eisherton, near Salisbury, in 

 beds that seem to correspond with the brick-earths associated with 

 the Coombe Pock, we find many species of high northern mammals*. 

 At Bovey Tracey, associated with the " Head " (a deposit very like 

 Coombe Eock), the Arctic birch, bearberry, and some northern 

 willows occur. In Central Erance the Eeincleer is abundant. 



Judging from the northern character of the fauna and flora, the 

 mean temperature of Xorth-western Europe at this period cannot 

 have been less than 20 : lower than it is now. probably it was 

 about 30 c lower. This would give a mean temperature in the 

 south of England very considerably below the freezing-point ; con- 

 sequently all rocks not protected by snow would be permanently 

 frozen to a depth of several hundred feet. 



* Stevens, -'Flint Chips.' pp. 12-30. 



