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, 6). 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 



