98 VEGETATION OF THE PEAK DISTRICT [CH. 



often be inferred that the wood is on its way towards heath or 

 nioor; whilst where the ground vegetation consists largely of 

 wood soft-grass (Holcus mollis), the wood is more likely to be 

 converted ultimately into grassland. On the other hand, a 

 wood whose ground vegetation consists largely of shade-loving . 

 species, such as wood anemone {Anemone nemorosa) and wood- 

 ruff (Asperula odorata), shows no signs of degeneracy and is 

 probably in a more or less static condition. 



PROGRESSIVE AND RETROGRESSIVE SCRUB 



Retrogressive scrub is so-called because it results from 

 the degeneration of chief associations (see p. 21). Progressive 

 scrub, on the other hand, leads to the establishment of chief 

 associations. As has been stated, the latter type of scrub is of 

 uncommon occurrence in this district. However, small examples 

 of progressive scrub may be seen here and there. They are 

 found on fresh soils at the base of cliffs, on projecting ledges of 

 cliffs, on screes, and in deserted quarries. On the sandstones 

 and shales, in fact, they are almost limited to the last kind of 

 locality. Mr Margerison (1907 8) has published an excellent 

 account of the vegetation of sandstone quarries near Bradford, 

 Yorkshire ; and his account is of more than local interest. 

 Mr Margerison shows that the plant succession of some of the 

 disused sandstone quarries which he investigated has reached 

 the stage of a birch (chiefly Betula pubescens) wood ; and it is 

 possible that this stage may in time be replaced by a wood of 

 Quercus sessiliflora. On the limestone areas, however, the 

 culminating stage is an ash (Fraxinus excelsior) wood. Retro- 

 gressive scrub is usually open : progressive scrub is frequently 

 closed, and often forms dense impenetrable tangles of low 

 woody vegetation. 



COMPARISON OF THE TYPES OF RETROGRESSIVE SCRUB 



The different types of scrub of the district, then, are 

 related to chief associations of woodland. The decay of oak 

 and birch woods results in types of scrub which should be classed 

 in the same plant formation as the oak and birch woods ; and 

 the decay of ash woods results in types of scrub which should 



