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 1s 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 
