104 VEGETATION OF THE PEAK DISTRICT [CH. 



In a few places, however, as on the elevated summit of 

 Bleaklow Hill, at a height of about 2000 feet (610 m.), sub- 

 Alpine grassland occurs on ground which has probably never 

 been tree-clad — at least, not in post-Tertiary times. It will be 

 shown later on that such sub- Alpine grassland occurs, so far as 

 this district is concerned, on sites which were comparatively 

 recently covered with peat ; and the peat having suffered 

 denudation, plants of the siliceous pasture have successfully 

 invaded areas which were once peat-clad. 



Natural grassland is rather uncommon on the less elevated 

 plateaux, for these are usually either occupied by moorland 

 associations or they are under cultivation. 



Types of Grassland 



Two main types of grassland occur in the district. One is 

 developed on the siliceous soils, the other on the calcareous 

 soils. The former type of grassland is characterized by the 

 great abundance of heath-loving or humus-loving species, and is 

 termed siliceous grassland. The non-calcareous or siliceous soils 

 allow of the formation and accumulation of acidic humus in the 

 soil ; but any great excess of this is, on steep slopes, prevented 

 by the denuding action of rain and melting snow. Instead of 

 the accumulation of peat, we get, on steep slopes, a slow but 

 continuous exposing of new soils. Such conditions favour the 

 growth of sward-forming grasses rather than of heathy under- 

 shrubs, for although newly exposed siliceous soils are much 

 poorer in soluble minerals than calcareous soils, they are richer 

 than sour peaty soils. On the plateaux, however, the acidic 

 humus or peat may accumulate ; and the ground is then invaded 

 by heather (Calluna vulgaris) and ecologically allied species. It 

 seems certain that the steep shaly slopes will never become 

 peat clad, whilst the grassland of the non-calcareous plateaux 

 will probably be ultimately converted into moorland. 



The sub-Alpine pasture above mentioned is essentially 

 identical in its ecological and floristic characteristics with 

 siliceous pasture ; but, as it occurs at higher altitudes, it has 

 fewer associated species. All the species, however, which 

 actually occur on the sub-Alpine pasture, occur on heath 

 pasture also; and the two associations therefore are placed 



