188 VEGETATION OF THE PEAK DISTRICT [CH. 
Occasional or rare 
Deschampsia flexuosa Narthecium ossifragum 
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 (Vaccinium 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 Schréter (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 Hriophorum 
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—87), 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 (04 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 
