66 VEGETATION OF THE PEAK DISTRICT [CH. 



type, and to the association of ash-birch woods (cf. p. 40). In 

 Somerset (Moss, 1907 : 41), ash woods are well developed on 

 the slopes of hills of Carboniferous Limestone, of the Dolomitic 

 Conglomerate, and of the Jurassic limestones. The "oak-hazel 

 woods" of Somerset (Moss, 1907 : 51) also are to be regarded as 

 conforming to the general ash wood type (see Watson, 1909 ; 

 and Moss, Rankin, and Tansley, 1910: 138). "Oak-hazel 

 woods," i.e., woods with oak standards and much hazel coppice, 

 are of very general occurrence throughout southern England. 

 Some of them have been derived from oak (Quercus Rohur) 

 woods, and others from ash-oak woods. The "oak-hazel woods" 

 of Somerset must be referred to the latter class, because they 

 contain among the coppiced layer a great deal of ash, which 

 would spring up as standard trees if not coppiced, and because 

 their "ground Hora resembles the more shady portions of the 

 ash wood" (Moss 1907: 52). 



In the Peak District, typical ash woods occur on the slopes 

 of the hills of the Carboniferous or Mountain Limestone (see 

 figures 9, 10 and 17). None is represented in the northern 

 area, as there no limestone rocks occur. In the southern 

 area, they are well represented, especially in Wye dale and 

 I^athkill dale. 



The Carboniferous Limestone rocks of north Derbyshire form 

 a plateau which attains a height of about 1550 feet (472 m.), 

 and whose average height is perhaps 1200 feet (366 m.). The 

 plateau is dissected by numerous valleys or "dales," most of 

 which are streamless. The limestone dales of the Pennines are 

 comparable with the gorges and coombes of the Mendip Hills 

 of Somerset, both from the standpoint of the geology and that 

 of the vegetation. The dales of Derbyshire descend from the 

 plateau, and the ash woods begin to appear on the slopes at an 

 altitude of about 1000 feet (305 m.), above which altitude scrub 

 occurs, but no genuine woods. The woods continue to the 

 bottoms of the dales, which here descend to about 250 feet 

 (76 m.). This is much lower than any of the ash woods 

 or scrub on the Carboniferous Limestones of the mid-Pennines ; 

 and, as in Somerset, the lower altitude permits of a better 

 development of the dominant tree and the more characteristic 

 shrubs and ground species of the ash woods. 



It is probable that at some past time, the whole of the 



