98 VEGETATION OF THE PEAK DISTRICT [cH. 
often be inferred that the wood is on its way towards heath or 
moor; 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 
