V] ASSOCIATIONS OF ROCKS AND SCREES 139 



On the older screes, the plant associations tend to become 

 more and more closed ; and it is well known (cf. Warming, 

 1909 : 246) that screes often show a developmental history. In 

 this district, as in Somerset, three types of plant succession 

 may be recognised as characteristic of the screes. The most 

 frequent case is the succession which terminates in calcareous 

 grassland. A not uncommon succession terminates in an ash 

 wood, and intermediate stages of this succession are well shown 

 on screes in Haydale, east of Cressbrook Dale. The least 

 frequent succession leads on to a kind of limestone heath, 

 as at the head of Monksdale, north of j\Iiller's dale, where 

 Calluna vulgar^is occurs side by side with lime-loving plants : 

 Smith and Rankin (1903: 167) mentioned that a similar kind 

 of vegetation is seen on some of the limestone screes of the 

 mid-Pennines. 



The following list was compiled from older screes adjoining 

 an ash wood : — 



Phegopteris Robertiana Scrophularia nodosa 



Cystopteris fragilis Teucrium Scorodonia 



Poly])odium viilgare Galium sylvestre 



Corylus Avellaua Sambucus nigra 



Urtica dioica Campanula rotundifolia 



Thalictrum minus Valeriana sambucifolia 



Sedum acre Valerianella olitoria 



Saxifraga hypnoides V. carinata 



Rubus saxatilis Senecio Jacobaea 



Crataegiis Oxyacantha Solidago Virgaurea 



Geranium lucidum Picris hieracioides 



G. sangiuneum Arrhenatherum avenaceum 



G. Robertianum Brachypodium sylvaticum 



Oxalis Acetosella Melica nutans 



Mercurialis perennis Convallaria majalis 



Cornus sanguinea Allium ursinum 



On higher mountains than occur in Derbyshire, screes are 

 developed to a correspondingly great extent : and the stones 

 composing the screes may then be many yards in diameter. 

 The vegetation of such block-screes is usually extremely scanty, 

 as the large size of the boulders prevents so much light from 

 reaching the soil below that seedling plants are unable to 

 reach maturity. Such tracts are well known in the Alps, and 

 have been described by the Swiss plant geographers under 



