The Heath Association 103 



both on the Bunter sandstone. We do not know how 

 ^ong this association can maintain itself unchanged, but 

 there seems good reason to believe that it is in course of 

 degeneration to the heath association. 



Heath association (Callunetum arenosum). The heath 

 . association is a stable association, either 



resulting, as has been already explained, 

 from the further degeneration of oak-birch heath or 

 developed de novo on the poorer sands. The gravelly or 

 relatively coarse sandy soil on which it is nearly always 

 based is characterised by its dark humous surface layers, 

 and generally covered by a thin layer of relatively pure 

 dry peat giving an acid reaction. In some heath soils a 

 hard continuous layer (moorpan, Germ. Ortstein, Fr. alios) 

 occurs at a distance varying from some inches to a few 

 feet below the surface and often some inches in thickness. 

 This consists of sand or gravel bound into a compact 

 stratum either by humous compounds {Ortstein proper 

 of G-raebner) or by iron oxide (Baseneisenstein) . It is 

 formed at the junction of the surface layers of leached 

 sand (often paler in colour) and the unaltered sand below. 

 The shallowing of the soil caused by the formation of 

 moorpan is very inimical to the growth of trees, and 

 heaths and moors in which hard pan is present cannot be 

 afforested unless the pan is broken up or at least holes 

 are made in it for the roots of the young trees to penetrate. 

 Pan is sometimes formed in woodland soils which are 

 undergoing degeneration and may play an important part 

 in the process, but it does not occur in f^^^^^J^ 

 there seems no doubt that the P^-^^-^^j'Jl^'Z^Z 

 described above can take P^-^- f :," Tn ;he Hythe 

 of the heaths of ^-;^'^'Z^^^£^V^-toi pan is played 

 beds of the Y^^^JlSrfiei^L below the s^uriace 

 by a ^^^\f This limits the growth of the trees of the 

 "^^^^birch heath (particularly of the oaks, which are very 



