Conditions of Heath Formation 99 



rainfall exceeding 60 inches probably has a real signi- 

 ficance \ The Scottish heaths develop a deeper layer of 

 relatively pure acid humus, up to 8 or 12 inches (20 to 

 30 cm.) according to Hardy, and have much in common 

 with the Calluna moor of the Pennines (see p. 113). 



At the other extreme we have the East Anglian heaths 

 with an annual rainfall of 25 inches or less, and a minimum 

 of dry peat formation. Between these extremes come the 

 heaths of the south-eastern counties and those of the New 

 Forest and Dorsetshire, with a layer of dry peat seldom 

 more than a fraction of an inch in thickness passing' 

 down into sand darkened by humus. The surface layer 

 of dry peat is largely formed by lichens and mosses (e.g. 

 Cladonia rangiferiim,, Polytrichiom piliferum, and other 

 species) which are the pioneers in reconstituting the 

 heath association on bared soil, as can be well seen 

 where the vegetation is destroyed by digging for gravel. 



It is possible that the heath formation originally 

 colonised many tracts of the poorer English sands, e.g. 

 the Pliocene "crag" and some of the more recent Qua- 

 ternary sands and gravels of East Anglia and the south, 

 perhaps also the Eocene Bagshot sand. If so, these sands, 

 or at any rate parts of them, have never borne natural 

 woodland. 



But apart from this possibility there is no doubt that 

 the heath formation is constantly successfully invading 

 and eventually replacing the natural woodland of many 

 sandy soils. The causes and course of this process have 

 already been briefly discussed. 



Within the dry oakwood association, more or less 

 isolated patches of heath vegetation are often found, 

 with bilberry (Vaccinium Myrtillus), the characteristic 

 heath grass Deschampsiaflexuosa, and in well-lighted spots 

 the ling (Calluna vulgaris). Though sometimes, no doubt, 

 the occurrence of these plants is dependent on local 

 I See Hardy, 1905 : p. 103. 



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