176 



Sub-formation of the, Chalk 



Climatic 

 conditions. 



rise to a curious sub-association which may be called 

 chalk-heath, parallel with the limestone heath (p. 158). 

 ^ Very characteristic of chalk pasture are numerous 

 species of Orohidacese, some of which are extremely rare. 

 The exposure of the chalk pasture association to the 

 atmosphere and to radiation is extreme. 

 There is little or no shelter from the full 

 eif act of sunlight, wind, rain and radiation. 

 The very thin soil is able to retain very little water, such 

 aerial water as is not quickly evaporated rapidly passing 

 down into the chalk below. On some chalk summits and 

 steep slopes the soil almost disappears owing to its 

 " creep " to lower levels, and the vegetation becomes 

 open. During protracted summer droughts much of the 

 herbage commonly withers, but the deeper rooting plants 

 are to some extent protected by thick underground organs 

 and sometimes by xerophilous adaptations. 



The following is a list of species occurring in chalk 

 pasture. The species clearly belonging to the heath 

 association, sometimes developed on the non-calcareous 

 soil overlying the chalk, are excluded : — 



Dominant — Festuca ovina. 



