194 VEGETATION OF THE PEAK DISTRICT [cH. 
through the retrogressive associations of bilberry, and, as regards 
the moorland formation, ended with bare peat. This open phase 
of a retrogressive series of changes terminates a formation, just 
as, in a progressive succession an open association is the starting 
point of a formation. In the case under consideration, however, 
the succession has not ended with the terminal association of 
the moorland formation, but has been continued by an open 
grassland association. What the future of this succession will 
be is a matter of speculation; but one may easily imagine, 
assuming climatic conditions to remain unaltered, that the 
future of this succession will show a closed association of grass- 
land, of mixed grassland and heath, and later, in the still more 
distant future, of some moorland association. 
The matter of this succession has been discussed here at 
some length in order to show that it is possible to account for 
the changes which the moorland vegetation has in comparatively 
recent times undergone, on other than climatic grounds. 
ZONATION OF THE MOORLAND AND GRASSLAND ASSOCIATIONS 
The zonation of the moorland and grassland associations of 
the non-calcareous summits and slopes of the southern Pennines 
is exceedingly well marked, as a study of the vegetation maps 
will themselves testify. In a general way, the zonation of plant 
associations on any mountain illustrates the effects of altitude 
on vegetation; but these effects are usually modified to some 
extent by some local conditions. In this district, the local 
conditions which compete with altitude in modifying the zona- 
tion are chiefly physiographical in character. However, the 
combined effects may be stated in general terms, if one speaks 
of the broad outlines of the vegetation and ignores details. 
Those eminences which are capped by a fairly flat plateau 
are characterized by summits which are covered with retro- 
gressive moorland associations (figure 33,a). These retrogressive 
associations are surrounded by rocky escarpments covered with 
stable associations of bilberry (figure 33, b). More pointed 
eminences are capped by a stable bilberry moor (figure 34, b). 
The bilberry moors are surrounded by moderately elevated, 
shelving plateaux of only moderate steepness; and on these 
plateaux a broad zone of cotton-grass moors (figures 33 and 
