164 VEGETATION OF THE PEAK DISTRICT [cH. 
and usually dominated by heather (figures 33 and 34): the peat 
of the higher plateaux is much deeper and usually dominated 
by the cotton-grass (figure 33c). This typical regularity of the 
moorland plateaux is, however, frequently broken by alternating 
outcrops of sandstone and shale. The outcrops of sandstone are 
usually characterized by a preponderance of bilberry (figure d), 
and those of the shale by swamps in which the larger rushes 
(Juncus effusus and effusus forma compactus) are generally 
conspicuous. The vegetation of these Juncus swamps has 
already been described (see pages 146—150). Sometimes, even 
in the general moorland area, a steep, shaly hill-slope is 
characterized by siliceous pasture. 
The moorland vegetation ascends to the highest summits 
of the district, three of which attain an altitude slightly 
exceeding 2000 feet (610 m.). No Arctic or Alpine species 
eccur anywhere on the southern Pennines, as in the case of 
the higher Pennine summit of Cross Fell further to the north 
(Lewis, 1904a: 328; 1904b: 279). The vegetation of the 
highest plateaux of the Peak District is marked by the 
occurrence of retrogressive moors (figures 28 to 31) with much 
bilberry (Vacciniwm Myrtillus), which very often alternates 
with patches of crowberry (Hmpetrum nigrum), less frequently 
of cloudberry (Rubus Chamaemorus), and sometimes of bare 
eat. 
: The altitude to which the moorland vegetation descends is 
determined partly by physiographical and partly by artificial 
causes. Where the moorland ceases abruptly and is separated 
merely by a stone wall from the permanent pasture of the 
upland cultivation (see figure 24), the lower limit is simply 
the place where reclamation has stopped; and this limit usually 
varies from about 1000 feet (805 m.) to about 1600 feet (488 m.); 
but where the moorland adjoins siliceous pasture, the limit is 
a perfectly natural one. The latter limit varies froth about 
750 feet (229 m.) to about 1500 feet (457 m.). Where the 
moorland descends to the lower of these altitudes, a zone of 
heather moor invariably occurs ; and where the moorland ceases 
at the higher of these altitudes, this zone is absent. 
In ascending the lateral slope of an upland valley, the change 
from the grassy slope to the peaty plateau is rather abrupt, 
and the transition region narrow. This is well seen in an 
