A HISTORY OF SUSSEX 



Devon they yield arctic plants. Judging from the northern character 

 of the fauna and flora, the mean temperature of north-western Europe 

 at this period cannot have been less than 20° lower than it is now — 

 probably it was 30° lower. This would give a mean temperature in the 

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

 quently all rocks not protected by snow would be permanently frozen 

 to a considerable depth. This would modify the entire system of drain- 

 age of the country. All rocks would be equally and entirely impervious 

 to water, and all springs would fail. While 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 imme- 

 diately 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, and 

 would deposit this rubble on the low lands, where the slope becomes 

 less and the streams had room to spread over fan-shaped deltas of the 

 material thus brought down. 



One result of this exceptional type of valley erosion is seen in the 

 peculiar way in which the heads of the coombes almost touch but do 

 not breach the escarpment. Coombe after coombe can be followed 

 upwards till its slope suddenly steepens and it ends abruptly in a sort of 

 ' cirque.' If the terminal wall of this cirque is cHmbed the sudden drop 

 of the escarpment is seen just beyond ; so that one can walk for miles 

 along the edge of the Downs on a gently undulating ridge, which is 

 often so narrow that from the same point a stone can be thrown down 

 the escarpment on the one hand and into the coombe on the other. 

 This shows that the escarpment has not been cut back since the coombes 

 were formed, and it also shows that the coombes were formed when the 

 escarpment had already receded to its present position. The abrupt 

 ending of each coombe is obviously connected with its having cut back 

 to a point beyond which, owing to the proximity of the escarpment, 

 there could be no erosion, owing to the absence of any gathering ground 

 for the rain. 



The Downs are dead. Their flowing outlines and winding valleys 

 point to bygone conditions, which can never recur till our climate again 

 becomes arctic. The only noticeable change now going on is the 

 gradual accumulation in the valleys of flints dislodged by sheep from 

 the steep slopes above. Another less obvious change is caused by the 

 gradual dissolving away of the Chalk by rain that falls on its surface. 

 This is a slow process ; but its tendency is to transform fertile Down into 

 stony waste, through the accumulation beneath the turf of a thicker and 

 thicker mass of the indestructible flints. 



Before quitting the open Downs, with their short sweet pasture 

 and easily worked soil, so valuable when most of the country was forest, 

 reference should be made to a curious relic of the ancient settlers. At 

 the foot of the Chalk hills is often found a thick bushy hedge, which can 



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