176 VEGETATION OF THE PEAK DISTRICT [CH. 
local rainfall is characterized by a well-developed and typical 
‘heather moor. Again, whilst the vegetation maps of districts 
north of the Peak might, if taken by themselves, be held to 
indicate that the heather moors were characteristic of the 
eastern and not of the western Pennines, it will be seen that 
that indication is not borne out by the vegetation maps of the 
present district. The comparative dryness of the peat of the 
heather moors is to be explained, not by rainfall statistics, but 
by the larger proportion of silica mixed with the peat of the 
heather moors, and by the greater shallowness of the peat. 
HEATHER Moors 
Associations dominated by the common heather (Calluna 
vulgaris) are among the most typical plant associations of the 
British Isles. In a general way, such associations may be sub- 
divided into heaths and heather moors, the former occurring on 
soils containing a higher proportion of sand and which are 
therefore drier, and the latter on soils containing a higher 
proportion of acidic humus and which are therefore wetter. 
In general, heaths are characteristic of the south and east of 
Britain, heather moors of the north and west. Hence, the 
occurrence of heath and of heather moor in this country would 
seem to be determined to some extent by climate. The 
associations of Calluna vulgaris (see figure 22), which so often 
occur as a fringe of the Pennine peat moors, are, on the whole, 
heather moors and not heaths, though some of those found at 
lower altitudes approximate in character to heaths, 
Weber (1908: 91) suggests that the term moor should be 
used only when the peat is 20 cm. or more in thickness and 
when there is less than forty per cent. of ash [including silica] 
in the peat; but a too rigid use of these criteria lead to an 
artificial classification. 
Beginning at Hayfield, what may be called the western 
system of heather moors extends northwards for about ten miles 
(16°1 km.), their continuity being broken by the narrow but deep 
clough formed by Shelf Brook, and by the larger Longden- 
dale, formed by the river Etherow. The northern slopes of 
