188 VEGETATION OF THE PEAK DISTRICT [CH. 



Occasional or rare 



Deschampsia flexuosa Narthecium osaifragum 



Nardus stricta Pinguicula vulgaris 



Moors of this transitional type have been described as 

 occurring on all portions of the Pennines which have been 

 investigated. 



Retrogressive Moors 



The bilberry ( Vacdniuni Myrtillus), in addition to its being 

 the chief plant on the screes, edges, and ridges of the sandstone 

 rocks, also becomes exceedingly prominent on the peat which 

 is in process of denudation on many of the highest watersheds 

 and plateaux. 



Woodhead (1906 : 351) appears to think that the occurrence 

 of the bilberry may perhaps always indicate the site of former 

 forest ; and he quotes Friih and Schroter (1904) almost to the 

 same effect. This opinion, however, is scarcely applicable to 

 the Pennines where the bilberry occurs abundantly in situations 

 which do not resemble its Alpine habitats. 



Whilst the peat of the closed association of Eriophorum 

 vaginatum is still increasing in thickness at a comparatively 

 rapid rate, and that of the closed associations of heather and 

 bilberry is also increasing though much more slowly, the peat 

 on the most elevated portions of the moors is gradually being 

 washed away. This process of physical denudation represents 

 a stage through which, it would appear, all peat moors, if left to 

 themselves, must eventually pass. Following Cajander (1904: 1 

 and 35 — 37), the associations thus formed are termed retro- 

 gressive [" regressive "] associations. 



In the Peak District, the process of retrogression in the 

 cotton-grass moors is apparently initiated by the cutting back 

 of streams at their sources. For example, the streams on the 

 Peak are shown, on the revised Ordnance survey maps (1870 — 

 1880), to be nearly three-quarters of a mile (1*2 km.) longer 

 than they were when the Peak was originally surveyed in 

 1830; and they are now a quarter of a mile (0'4 km.) longer 

 than they are shown to be on the revised maps of 1879. The 

 channels formed by the streams which have thus eaten their 



