VALLEYS DUE TO STRATIFICATION. 139 



we have the valley occupied by the English Channel or the Thames, 

 and the still smaller valleys which branch about and f oriii glens in the 

 mountains of Scotland, or cwms in Wales. Almost every country 

 offers examples of valleys through which the streams and rivers find 

 their way to the sea, which differ from each other considerably in 

 their scenery and origin. Since they are all formed by (i) the opera- 

 tion of the forces of compression which bend the rocks into folds, or 

 by (2) the action of rain or ice or the sea which fashion and carve 

 the irregularities of the earth's surface, or are due to (3) the alter- 

 nation of the different kinds of rock which make the geological 

 structure of the country, it is convenient to consider valleys according 

 to the ways in which they have originated. 



Valleys of Stratification. In the middle districts of Britain, 

 running from Yorkshire towards Dorsetshire, the several Secondary 

 strata, which are chiefly alternations of limestones and clays, rest 

 successively upon each other, tilted up at an angle, so that the several 

 beds dip into the ground to the south-east. A clay is usually contained 

 between two beds of limestone. The clay being formed of impalpable 

 mud, has its surface particles loosened year by year under the influence 

 of frost, and day by day by falling rain, which sweeps along the 

 loosened particles, holding them in suspension, and delivers them to 

 a river, which carries the waste material to the sea. Thus, in time, 

 the clay becomes hollowed out into a valley more or less deep and 

 broad, while the limestone, which is less easily broken up by the frost, 

 and has few loose particles which can be carried away by moving 

 water, and is only slowly dissolved by the carbonic acid which rain 

 brings to the earth from the air, wastes less rapidly ; and hence the 

 limestone stands up as a terrace, margining the valley hollowed out 

 in the clay below it. Such valleys are formed between the Chalk 

 range of the North Downs and the parallel ridge of the Lower 

 Greensand which extends from Guildford eastward by Leith Hill. 

 The clay worn away is the Gault ; the valley is the Holmedale. A 

 similar valley lies between the range of the Lower Greensand and the 

 parallel range of the Tunbridge Wells Sand in this Wealden area. 

 It is formed by excavation in the Weald clay. A third valley is 

 excavated parallel to the others between the Tunbridge Wells Sand 

 and the Ashdown Sand, which forms the central boss or saddle of the 

 Wealden anticlinal fold. This series of valleys, more or less distinctly 

 marked, is repeated with a repetition of the strata southward, between 

 the Ashdown Sands and the English Channel. So that in the little 

 area of the Weald of Kent, Surrey, and Sussex there is a system of 

 six parallel valleys which run eastward to the sea. The whole of the 

 geological structure of the island offers repetitions of the same pheno- 

 mena. The Vale of Pickering, in Yorkshire, drained by the river 

 Rye, coming down from the Yorkshire moorlands, and by the Derwent, 

 is hollowed in the Kimmeridge clay, between the terrace or escarp- 

 ment of the Coral Rag and the Chalk. Similarly, in the south of 

 England, in passing from Swindon to Bristol, valleys in the Kimmeridge 

 Clay, Oxford Clay, and the Lias are crossed. 



