CH. IIT] SCRUB ASSOCIATIONS 89 
It is clear, therefore, that the upper altitudinal limit of woods 
or of trees in any particular district cannot be accounted for 
merely by the factors connected with the altitude of that 
district. This point of view, although quite an old one, is 
frequently ignored. 
Smith (1911: 13) says that “tree growth ceases wherever 
the wind attains such a force the formation of young shoots is 
prevented. The determination of this wind zone must always 
be considered in forestry. It cannot be defined as so many 
feet above sea-level. The action of the wind may be very 
marked on the coast itself....It is also the case in a hill-mass, 
that all the zonal limits of plants are lifted up.” This is indeed 
quite true, and helps to explain the occurrence of woodland plants 
under the peat of Cross Fell (which rises to 2930 feet = 893 m.) 
at an altitude of 2400 feet (731 m.), whilst under the peat of 
the Peak of Derbyshire (which only rises to 2088 feet = 636 m.) 
woodland plants have not, during the course of the present 
investigation, been observed higher than 1800 feet (549 m.). 
It is not necessary to invoke post-glacial climatic changes to 
account for apparent discrepancies of distribution of this 
nature, for they can easily be paralleled by similarly apparent 
discrepancies in existing vegetation. 
Warming (1909: 39) has stated that trees cease on 
mountains at the altitude where they break up into separate 
peaks. It seems highly probable that at the conclusion of 
the glacial period, this country was invaded by Arctic-Alpine 
species, and afterwards by forest or woodland species. Lewis 
(1905, etc.) finds two forest layers in some of the Scottish peats, 
so that perhaps there were two separate invasions of forest 
plants. However, only one such layer appears to be represented 
as a rule in the peat of the Peak District, though in other parts 
of England two or more layers of trees are found buried by peat. 
The primitive woods probably ascended the mountains up to or 
nearly up to the limit indicated by Warming. Perhaps there 
was above this primitive forest limit, a narrow girdle of climatic 
scrub and a still higher girdle of climatic grassland; but, as 
regards the present district, this is not certain. In any case, 
from that time to this, there has been a gradual lowering of the 
forest limit; and the scrub and grassland, which now characterise 
the higher slopes of the district (and indeed those of Europe 
