Distribution of Calcareovis Ashwoods 147 



Wales and in the Mendip Hills in Somerset^ and is ex- 

 posed over considerable areas in Ireland. The vegetation 

 of this rook has been principally studied in the Pennine 

 area and on the Mendips, and the following account is 

 mainly based on these regions. 



As in most other cases, the characteristic vegetation 

 of the older limestones is naturally considered as falling 

 into three associations corresponding to woodland, scrub 

 and grassland. 



Ashwood association (Fraxinetum excelsioris) 

 (Plates XIII and XIV). 



Ashwoods are characteristic of the limestone hills of 

 the north and west of England. Woods of this type 

 occur on the limestone hills of Yorkshire' and Derbyshire^, 

 Westmorland'* and Somersef. 



Those described by Smith and Rankin are in many 

 cases of the nature of scrub (see p. 153)^ and 

 those described by Lewis are of the ash-birch 

 type (see p. 149). In Somerset, ashwoods are well 

 developed on the slopes of hills of the Carboniferous or 

 Mountain Limestone, of the Dolomitic Conglomerate, and 

 of the Jurassic limestones. 



In Derbyshire, they are well developed on the slopes 

 of hills of the Mountain Limestone. The summits of the 

 limestone hills of north Derbyshire attain a height of 

 about 1550 feet (c. 470 m.), and their average height is 

 about 1200 feet (c. 370 m.). The plateau is dissected by 

 numerous valleys or " dales," most of which are streamless. 

 The dales descend from the plateaux, and the ash woods 

 begin to appear at an altitude of about 1000 feet (305 m.), 

 above which scrub occurs but no genuine woods, and 

 continue to the bottoms of the dales. 



1 Smith and Eankin, 1903. ^ Moss, 1911. 



3 Lewis, 1904 a. * Moss, 1907. 



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