Influence on biodiversity
Although limestones comprise only a comparatively small proportion of the district’s geological succession, their outcrops may be distinctive. Outcrops of limestone, where free, or substantially free, of superficial cover, typically support areas of species-rich limestone grassland. The brighter green vegetation on the limestone, stands out from the whiter vegetation of the more acidic soils developed on the intervening mudstones and sandstones, and may be a useful clue to identifying limestone outcrops. Typical species include eyebrights, salad burnet, thyme, rock rose and fine grasses such as quaking grass and bents and a variety of small sedges.
Quarrying, especially of the Great Limestone has left a legacy of abandoned quarries which exhibit varying degrees of degradation and regeneration and are commonly hosts to a rich limestone flora. Weathered limestone, in natural outcrops and abandoned quarries, provides extremely important habitats for a number of specialised plant communities, including as a substrate for ferns, lichens and other lower plants. Where limestone dissolution has taken place by natural groundwaters, emergent springs may carry a substantial amount of calcium in solution. Such springs may be associated with the development of tufa mounds and terraces in which calcite deposition may in part be facilitated by the influence of bacteria or algae. At least one calcareous spring near Kielder is associated with the development of flattened ovoid oncolites of algal origin. Calcareous springs may also raise the pH of stream or pond water and the surrounding wet grassland supports species such as devil’s bit scabious, grass of Parnassus, butterwort and fragrant orchid.
Both limestones and sandstones locally form areas of small bare crags and may provide important habitats for a variety of plants. Natural crags, cliffs and quarry faces may offer important nesting and roosting sites for birds and possibly bats.
The bulk of the Stainmore Formation comprises shales and sandstones which typically weather to a range of mainly acid soil types. The wide outcrops of these rocks are distinguished by expanses of moorland vegetation, including some fine examples of heather moorland, though management and grazing regimes have resulted in the widespread development of Nardus and Molinia grassland, known locally from its distinctive pale colour in the winter months, as ‘White lands’. Again Quaternary deposits, particularly peat deposition, influence the biodiversity more locally.