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 24
