46 VEGETATION OF THE PEAK DISTRICT [CH. 
stable association occurs in which the two species are present 
in almost equal proportions. Such transitional woods may 
occur in localities where the two associations come into close 
propinquity, and also sometimes on dry sandy or gravelly 
soils. In such transitional woods, the hybrid oak (Quercus 
Robur x sessiliflora) invariably occurs. The occurrence of this 
hybrid, though quite general in such situations, was unsuspected, 
so far as this country is concerned, until quite recently (see 
Moss, 1910a: 34). 
Oak Woops OF QUERCUS SESSILIFLORA 
Woods of Quercus sessiliflora occur on the damp, shallow, 
siliceous soils of the Coal-measures, the Millstone Grit, and the 
' Pendleside (or Yoredale) rocks, up to an altitude of about 
1000 feet (805 m.). The rocks of the Coal-measure series, as 
a rule, occur at moderately low altitudes, flanking the Pennine 
watershed. On these rocks, the woods, whilst mainly confined 
to the slopes of the hills, occasionally extend some little distance 
on to the plateaux, whereas on the rocks of the Millstone Grit 
and the Pendleside series, the woods are almost entirely confined 
to the steep slopes of the narrow valleys or “cloughs” ; and it 
is only rarely that they spread out on to the flatter and more 
exposed plateaux. 
The woods of Quercus sessiliflora of the southern Pennines 
have been described by previous writers. Crump (1904: xxxiil) 
subdivided them into (a) mixed deciduous woods, and (6) dry 
oak woods on the Coal-measure, Millstone Grit, and Yoredale 
(or Pendleside) rocks. Smith and Moss (1908: 387) and Smith 
and Rankin (1903: 159) adopted almost the same subdivisions 
when they described (a) lowland oak woods and (6) upland 
oak woods. Woodhead (1906), in describing their ground 
vegetation, referred to them as (a) mixed deciduous woods 
of the Coal-measure area (p. 336), and (b) mixed deciduous 
woods of the Millstone Grit area (p. 347). However, all these 
subdivisions are only particular aspects of the association of 
Quercus sessiliflora as developed on shallow, siliceous soils. 
The “pine woods” of Smith and Rankin (Joc. cat.) are merely 
plantations of conifers, on sites previously occupied by woodland, 
grassland, or farmland. 
