44, VEGETATION OF THE PEAK DISTRICT (CH. 
The woods of Quercus sessiliflora and of Betula pubescens 
are confined to the sandstones and shales. The soil over the 
sandstones is shallow, that is, shallower than would appear to 
be necessary for the free growth of the tap roots of Quercus 
Robur ; and similarly the soil of the shales is frequently shallow, 
being often interrupted by bands of flagstone and half-formed 
sandstone. The restriction of woods of Quercus sessiliflora to 
shallow, siliceous soils is very general throughout the whole of 
the British Isles; but exceptional cases occur on sandy and 
gravelly soils in the south-east of England. 
3. The altitude of the woodland site. Woods of 
Quercus sessiliflora cease at altitudes of about 1000 feet (305 m.), 
whilst woods of Betula pubescens reach altitudes of about 
1250 feet (381 m.). As the physical and chemical conditions 
of the soils of the two associations are identical, it seems 
clear that the failure of the oak woods to reach the higher 
altitude is due to the severer climatic conditions which prevail 
there.. Such conditions are the lower temperatures, the greater 
velocity of the wind, and (to some extent) the lower atmo- 
spheric humidity and higher rainfall. 
Oak Woops oF QuzRcUS ROBUR 
In this district, woods of the peduncled oak (Quercus Robur) 
occur only on the western boundary; and they represent the 
eastern fringe of the lowland oak woods of the Cheshire plain. 
The latter, in their turn, are a north-western continuation of 
the lowland oak woods which are characteristic of the Midlands 
and of the south of England; and an eastern extension of the 
latter may be traced through Nottinghamshire and into east 
Yorkshire. As already indicated, oak woods of Quercus Robur 
may occur either on deep clays or on deep sands or gravels. 
Woods of both these types occur on the Cheshire plain, though 
the oak woods of the sandy and gravelly soils are there more 
extensive than those on clayey soils. It is the woods of the 
former type that just reach the western margin of this district. 
The best examples of woods of Quercus Robur in this 
district occur in the lower courses of the rivers Etherow and 
Goyt, two of the head-streams of the river Mersey. Ernocroft 
Wood and Bottoms Hall Wood are examples of such woods, 
