152 ORIGIN OF SCENERY OF BRITAIN 



some instances, such as the flats of the Weald Clay 

 and the Chalk of Salisbury Plain, there is hardly any 

 such cover of detritus, the denuded surface of under- 

 lying rock forming the actual floor on which the 

 vegetable soil rests. 



Our plains, if classed according to the circumstances 

 of their origin, may be conveniently regarded as (i) 

 river plains strips of meadow-land bordering the 

 streams, and not infrequently rising in a succession 

 of terraces to a considerable height above the present 

 level of the water ; (2) lake plains tracts of 

 arable ground occupying the sites of former lakes, 

 and of which the number is ever on the increase, 

 owing to the filling-up of the basins with sediment ; 

 (3) plains consisting of portions of upraised sea-floors 

 partly eroded rock-platforms, but mostly flat sel- 

 vages of alluvial ground, formed of littoral materials 

 deposited when the land lay below its present level : in 

 the northern estuaries these raised beaches spread out 

 as broad carse-lands, such as those of the Tay, Forth, 

 and Clyde ; (4) glacial drift plains tracts over which 

 the clays, sands, and gravels spread out during the 

 Ice- Age form the existing surface ; (5) plains of sub- 

 aerial denudation which have been levelled by rain 

 and other atmospheric agents, especially upon tracts 

 of rock of fairly uniform resistance, such as the soft 

 clays and sands of the Secondary and Tertiary forma- 

 tions ; (6) submarine plains the present floor of the 

 North Sea and of the Irish Sea, which must be 

 regarded as essentially part of the terrestrial area of 

 Europe. 



When plains remain stationary at low levels, they 



