Rocks associated with lowlands
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1.1 Lichens can be conspicuous inhabitants of rocky habitats. These can be natural outcrops or erratic boulders, or they can be man-made exposures in quarries, rail and road cuttings, etc. Further lichen communities can exist on rockery stones and decorative boulders in gardens and estates. Lowland rock exposures are important for extending the geographical range for many lichen species. In particular, lichens common in the north and west of Britain are enabled to inhabit small areas of south and east Britain where lowland rock exposures are present. Man-made rocky substrata further enable extension of the range of many lichens (see this volume, chapter 14). However, many lowland rock exposures are scarce or of very limited area and are deserving of publicity and protection.
1.2 Lowland rocks have seldom been specifically written about, though information can be gleaned from Lichenological field excursions reported in The Lichenologist. Detailed reports occur in Pentecost and Rose (1985) dealing with the Wealden sandstones, Hawksworth (1969) in the Lichen Flora of Derbyshire, Bowen (1976) for Dorset, Fletcher (1979) for parts of Leicestershire. James et. al. (1977) gave a useful summary of lowland rock lichen communities.
1.3 These lowland rock lichen communities may be regarded as natural when they have developed over long periods of time on naturally exposed outcrops. Communities on man-made exposures are also important, and can probably be regarded as semi-natural as the lichens here are opportunists taking advantage of mans' provision of appropriate substrata.
1.4 Although this chapter is devoted to lowland rocks, they may occur in the highland zone, north and west of the line between the rivers Tees and Exe). However, it could be expected that the environmental factors and conditions influencing low-altitude rock lichens in the 'Highland' zone may be somewhat different to those in lowland Britain. This would be particularly true of temperature and rainfall.
2. Types of Lowland Rocks
2.1 Natural rock habitats. It is unusual for a rocky site to be entirely devoid of lichens. Bare rocks still occur in the air-polluted Midlands and industrial North, where rock exposures remain blackened by soot which seems to inhibit lichen colonization. Many lowland sites have rocky cliffs resulting from natural phenomena such as water erosion. These may be low outcrops, as in the Sussex Weald and Derbyshire edges, or can achieve great heights in the Avon, Cheddar and Derbyshire gorges.
Many on the Pennine fringes occupy hill tops, as in Charnwood Forest, Shropshire and Staffordshire, where they exhibit post-glacial frost shattering, and have many crevices and overhangs available for exploitation by lichens. Glacial-erratic boulders are frequent in the highland north and west, and on lowland sites in Wales and the South West, but tend to be buried by glacial drift in the lowlands. Prehistoric structures such as Henges, and natural Sarsen stones, because of their great age, have lichen populations indistinguishable from those on natural exposures and are often dealt with as natural outcrops. Shingle lichens occur on river beds and seashores.
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