162 Sub-formation of the Chalk 



The distribution of the chalk and some of its 

 characteristic agricultural features have already been 

 described in Part I (pp. 53-5). Stratigraphically and 

 lithologically the rook is divisible into three, the Upper, 

 Middle and Lower Chalk. The Upper Chalk is the 

 thickest division and also the purest, frequently con- 

 taining as much as 98 7„ of calcium carbonate. The 

 Lower Chalk is much less pure, containing considerable 

 percentages of clay and siliceous material generally. The 

 Lower and Middle Chalk are generally under cultivation, 

 and it is mainly the Upper Chalk which bears the 

 characteristic woodland, scrub and grassland to be here 

 described. A large proportion of its surface is, however, 

 covered by various later deposits of no great depth, such 

 as clay-with-flints, plateau gravels, and in the regions 

 near the North Sea, chalky boulder clay, together with 

 rainwash from these accumulations. The typical chalk 

 vegetation is thus generally confined to the escarpments 

 and valley sides where the actual chalk comes to the 

 surface. 



The vegetation of the chalk proper, as in the case of 

 Vegetation ot^er plant-formations, shows the obvious 

 division into woodland, scrub and grassland 

 associations with which we have now become familiar. 

 In many respects these associations are distinctive as 

 compared with the corresponding associations of the older 

 limestones. Thus the beechwoods of the escarpments and 

 valley sides of the chalk contrast with the ashwoods of 

 corresponding situations on the older limestones. To 

 this generalisation there are indeed exceptions. The 

 extreme limits of the chalk in Devon, Dorset and the 

 Isle of Wight bear ashwoods in place of beechwoods 

 and on the northern wolds of Yorkshire and Lincoln- 

 shire natural beechwoods are also absent; while the 

 oolitic limestones of the Cotswold Hills in Gloucestershire 

 bear well-developed natural beechwoods. 



