558 T. Mellard Reade — British Geology, 



Facts of British Geology. 



An examination of a good geological map of England and Wales, 

 such as that known as Greenough's, published by the Geological 

 Society, shows at once that the older rocks from the Cambrian 

 to the Carboniferous constitute the bulk of the more essentially 

 mountain areas. 



Thus, North Wales is mainly Cambrian and Silurian ; Cumberland 

 and Westmoreland are largely composed of equivalent rocks, sur- 

 rounded with a fringe of Carboniferous, which, more greatly developed 

 in Northumberland, Durham, Yorkshire, and Derbyshire, forms the 

 mountainous district of the Pennine Chain, the so-called backbone of 

 Northern England. Leaving out of consideration for the moment 

 the pi'e-Cambrian rocks of Anglesey and Shropshire, it is in the 

 Cambrian and Silurian that the greatest deformations and foldings 

 have taken place. Lying conformably upon the Upper Silurian in 

 Herefoi'd shire and Brecknockshire is the great thickness of Old Red 

 Sandstone, more horizontally developed, but yet in the Beacons of 

 Brecon and the Fans of Carmarthen rising as mountains from two 

 to three thousand feet high. These are overlaid conformably by the 

 South Wales Carboniferous rocks. In the North the Carboniferous 

 Limestone lies unconformably upon the Silurian, the Old Red being 

 practically absent. These Carboniferous rocks are in places con- 

 siderably folded, the Pennine Chain itself being an anticlinal axis, 

 while the Devonian and Carboniferous in Somerset and Devon are 

 folded often at high inclinations in the Quantocks and the Mendips. 



Surrounding these anticlinal areas, and in the depression between 

 them, the Permian Limestone breccias and sandstones and the 

 Triassic sandstones and marls lie unconformably upon the Carbon- 

 iferous rocks, and much of this country is hilly, but cannot be 

 correctly designated mountainous. The whole of the group of rocks, 

 from the Cambrian to the Trias, occupy, roughly speaking, the 

 western half of England, including Wales, but the eastern half of 

 England is constituted of a succession of younger foi'mations, from 

 the Rhgetic to the Pliocene. The Rha3tic constitutes the passage 

 from the Trias to the Lias, and, though nowhere developed on an 

 extensive scale, is interesting as showing a conformable succession 

 which is unbroken through the Oolites to the Upper Cretaceous. 

 Between the Cretaceous and Eocene is an unconformable break 

 representing a great time interval separating the Cretaceous and 

 Tertiary, which is further confirmed by the complete change in the 

 fauna and flora. The Eocene, Miocene, and Pliocene are conform- 

 able, and with them ends what is called the solid geology of the 

 country, the succeeding deposits being Pleistocene, Glacial, and 

 post-Glacial, consisting mostly of clay, sands and gravels. Boulder- 

 clay and alluvium. 



It is thus seen that the western half of England, together with 

 Wales, possess the mountainous districts par excellence, while the 

 eastern half is distinguished by gently undulating or horizontal 

 strata, with sharp folds located in small areas, as in the Isle of 

 Wight, at Purbeck, and the Yorkshire Chalk at Flamborough Head. 



