IX HERTFORDSHIRE. 181 



to the level of the sea by any subaerial action. "We may also 

 assume that the winter climate was more rigorous than it is at 

 present, and that the annual rainfall was much greater. At the 

 present time there is no geological formation less liable to floods 

 than the upper portion of the Chalk. It is so absorbent that under 

 ordinary circumstances it takes in the rain as fast as it falls upon 

 it, and the moistnre, when once in the soil, is either carried off 

 again by evaporation and vegetation, or descends to the plane at 

 which the chalk is saturated with water. This plane of saturation 

 varies much in different seasons ; near the Chalk escarpment in 

 Hex'tfordshire, at a spot several miles away from any stream, I 

 have known its level to vary as much as 70 feet in a single year; 

 but with a greater rainfall than at present the chalk might at all 

 times be saturated nearly up to its surface. Floods might then be 

 as readily produced as if the soil were the most unabsorbent of 

 rocks. 



As the land rose from beneath the retreating sea, shallow 

 channels might be formed by its waters, a course thus being 

 marked out along which streams would subsequently flow ; on 

 the bare surface of this newly-elevated tract the eroding power 

 of heavy rains would be great ; and with a rigorous climate there 

 would be a large accumulation of snow and ice in the winter, the 

 rapid thawing of which in the summer would add enormously to 

 the volume and the eroding power of the streams, causing them to 

 deepen and widen their channels. The valleys being at first broad 

 and shallow, the floods would cause the streams to overflow their 

 banks and spread over the bottom of the valleys, carrying with 

 them and depositing fine mud or clay, sand, small pebbles, larger 

 pebbles, and flints or other stones, according to the velocity of the 

 current. With each succeeding flood the valley would be deepened 

 and narrowed, the river would become less sinuous, and the 

 deposits spread by former floods over the bottom and the slopes 

 of the valley would gradually be left out of the way of subsequent 

 floods, the earliest-formed deposits being on the highest levels, 

 which first escaped from the disturbing action of the repeated 

 floods. Assuming, as T have done, that there were beds of marine 

 clay and shingle upon the surface of the chalk, there would be 

 in the higher and older gravels a much larger proportion of pebbles 

 derived from these beds than of flints from the chalk than would 

 be found in the lower and newer deposits ; for when the latter 

 formed the river would have worked its way below the level of 

 these upper beds, and it would be excavating the chalk and forming 

 the gravel in its bed chiefly from flints derived from the chalk. 



