MOUNTAINS. Sgi 



Plain of York, the North and South Clays (between Gainsborough and 

 Nottingham), and the Severn Valley are chiefly JVfia Red Sandstone. The 

 central plain of England is chiefly Liassic, with much Drift. Salisbury Plain 

 (400 to 600 ft. high) is Chalk. The Eastern Plain is chiefly Chalk covered with 

 Drift. The Lower Thames Valley chiefly Tertiary and Alluvium ; the Fenland 

 (Bedford Level, etc.), Somersetshire Levels (Sedgemoor, etc.), Romney Marsh, 

 Holderness, etc., chiefly Alluviuni and Drift. 



We see, therefore, that the age of the formations has to a large 

 extent affected the main features, and, in this country, at any rate, 

 we find a transition from the hills and mountains composed of 

 PalcEOzoic rocks to the Fens and Levels formed of Alluvium. 



Mountains, in the first instance, are produced by great folds 

 of the earth's crust, due rather to its contraction than to direct 

 upthrust ; and the materials composing them have been subject 

 to much mechanical alteration, exhibited in slaty cleavage, and 

 other forms of ' shearing.' They are often lines of weakness, being 

 associated with eruptive rocks, granitic masses and dykes of intrusive 

 material. Consequently, in many cases they have been subject 

 to repeated oscillations, although they have, on upheaval, often 

 continued to occupy the same prominent positions. Nevertheless, 

 while these grand features are due firstly to great lines of elevation, 

 their present forms have been shaped by denudation.^ 



Many years ago Sir Andrew Ramsay noticed that in constructing 

 a section across Wales, through the more hilly or mountainous 

 regions, a line might be drawn from one end to the other which 

 would touch, or nearly touch, all the more important elevations. The 

 whole of the rocks of Palaeozoic age which form these regions are 

 much disturbed or contorted, being bent into folds, and at the 

 same time irrespective of the shape of the hills. He demonstrated 

 that while filling up the valleys in imagination, there yet remained 

 a vast amount of material that had been removed above the plane 

 which touched the tops of the hills. This line indicated to him a 

 'Plain of Marine Denudation,' formed before the tract was elevated 

 to its present position, and probably in Carboniferous times during 

 a period of gradual submergence." 



Such plains of denudation are now in process of formation on 

 many parts of our coasts, as at Watchet or near the mouth of the 

 Thames at Southend, where the sea is destroying the cliffs and 

 leaving a platform of rock (whether stone or clay) barely covered 

 at low tide. The plain formed in Wales was of course a very 

 extensive one, and then, after the area was elevated, atmospheric 

 denudation came into play. The old disturbances in the rocks 

 may have partly affected the character of the plain of marine 

 denudation, which does not necessarily mean a dead-level, and 

 gentle curves or slight ridges of the harder rocks may have been 

 left as a guide to the agents of subaerial denudation.^ Moreover, 



^ See T. M. Reade, Origin of Mountain Ranges, 1886 ; A. Geikie, Mountain 

 Architecture, 1877 ; Murchison, Address to Geol. Soc. 1843, p. 78; Lyell, Ibid. 

 1850, pp. 24, 38; C. Callaway, G. Mag. 1879, p. 216; J. Geikie, Scottish 

 Geogr. Mag. 1886 ; see also Midland Naturalist, 1881, iv. 5. 



2 Mem. Geol. Survey, i. 297. ^ See also E. Hull, G. Mag. 1867, p. 569. 



