aS ae 
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 1s 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 Miller’s dale, where 
Calluna vulgaris 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 
Polypodium vulgare Galium sylvestre 
Corylus Avellana Sambucus nigra 
Urtica dioica Campanula rotundifolia 
Thalictrum minus Valeriana sambucifolia 
Sedum acre Valerianella olitoria 
Saxifraga hypnoides V. carinata 
Rubus saxatilis Senecio Jacobaea 
Crataegus Oxyacantha Solidago Virgaurea 
Geranium lucidum Picris hieracioides 
G. sanguineum 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 
