114 The Heath Formation 



grown high, woody and thinly leaved. They approach in 

 character the true Calliona moors (p. 275) of the Pennines, 

 in that they occur on a greater depth of surface peat than 

 the English heaths and contain more species characteristic 

 of the moor formation, especially where drainage becomes 

 impeded in hollows or on the flatter terraces. Neverthe- 

 less they are always developed, like the southern heaths, 

 over sand or gravel, and are not so damp (except locally) 

 as typical Galluna moor. We may consider that there are 

 two edaphic factors which control the heath formation, 

 the sandy or gravelly soil and the accumulation of acid 

 peaty humus, whereas the moor formation is controlled 

 by the acid peat alone. 



The thickness of the superficial peaty layer is commonly 

 4 to 8 inches (c. 10 to 20 cm.), though in extreme cases, 

 according to Hardy', it may reach a depth of 12 inches 

 (30 cm.). Below, the peat passes into sand or gravel, and 

 at a depth of 12 to 24 inches (30 to 60 cm.) from the 

 surface a layer of " moor-pan " frequently occurs. 



The topography of the Galluna heath varies to a 

 considerable extent, as it has a vertical range from the 

 valleys upward of 1500 feet (c. 450 m.), and its terrain 

 includes steep slopes, stream valleys, and undulating 

 summits. It is therefore natural to expect variation of 

 facies, and while Galluna is the dominant species, the 

 other associates vary considerably. Several of these owe 

 their outstanding position in the association to their quick 

 recovery after the moor-burning, so that they precede 

 Gallwna during the first year or two of regeneration. So 

 far as records are available, the following are species 

 which thus temporarily occupy the ground. Gladonia 

 rangiferina, and other species, often form almost the only 

 covering ; Erica cinerea recovers quickly on dry soils, 

 Vaccinium Myrtillus and V. Vitis-Idsea on steep slopes, 

 Arctostaphylos Uva-ursi at higher altitudes (towards 



i Hardy, 1905, p. 106. 



