470 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 24, 



perature would he the storing up of snow and ice, the sudden 

 melting of which in the spring would bring about floods. 



We have thus far been speaking of the rivers when the land is 

 stationary or sinking. If, however, an elevation takes place, the river 

 will commence deepening its channel. The elevatory action may be 

 so slow as to allow the river to travel all over its alluvial plain, 

 reducing all alike to a new level; but more commonly " terraces" of 

 the old alluvium will be left, which, unless completely removed by 

 atmospheric action, will remain to show the former position of the 

 river. This process we conceive to have been going on during a long 

 period of time in the Medway valley, the gravel at the 300-feet level 

 being the oldest river-bed remaining ; between which and the nearest 

 point of the Medway there is no higher ground intervening. 



B. On the Origin of Escarj)ments. — In treating this subject we will 

 first take a hypothetical case, and then apply the principles there 

 explained to the area under consideration*. 



Let fig. 11 represent in plan, and fig. 12 in section, three beds, 



A, B, and C, — A and C being sandstone, and B being clay; and let 

 us suppose the plane formed by the denuded edges of the beds to 

 slope down in the direction from A to C ; let rain faR on this 

 sloping surface, slight inequalities of the ground will make the 

 rain flow into a number of smaU rivulets, and, as the principal 

 slope is at right angles to the line of strike, the rivulets will take 

 the same general direction, and begin cutting out channels or 

 small transverse valleys. In plan, the channel would be shown 

 as in fig. 13. If we had nothing but sandstone of uniform hard- 

 ness, the stream would merely cut itself a gorge, the breadth of 

 which would be the same all along. When we come to rocks of 

 different hardness, however, the case is otherwise. The stratum 



B, being of clay, wiU suffer much more from atmospheric denuda- 

 tion at the sides of the gorge than the strata A and C. Each 

 shower of rain, each frost, will do its part in degrading the soft 

 clayey walls of the valley ; slips, too, may come to our aid, and the 

 transverse stream will carry off the debris and rain-wash. In this 

 manner the valley will be widened where it passes through the bed 

 B. Figs. 14 and 15 will show sections, alojig the lines vxr and 

 y z, through the sandstone bed and through the clay bed, before 

 the atmospheric agencies have had much action. Figs. 16 and 17 

 show similar sections through the two beds, after the denuding 

 powers of the atmosphere have produced some effect. The valley 

 on the clayey strata is widened considerably, whilst the walls of 

 the valley where formed by sandstone have scarcely suffered any 

 change. The result of atmospheric action wiU be that the walls 

 of the valley wiU get less and less steep where they are formed 

 by the bed B. A sort of amphitheatre will be formed on each 



* We would here again refer to tlie excellent paper by Mr. Jukes, in whicli 

 the connexion between longitudinal and transverse vaUeys was first clearly 

 explained, " On the River VaUeys of the South of Ireland," Quart. Journ. 

 Geol. Soc. vol. xviii. 1862, p. 378. See also Mr. Geikie's ' Scenery and 

 Geology of Scotland,' 1865, p. 138. 



