Drystone Walls

Drystone walls are usually assembled from the stones and rocks that are at hand and, therefore, are a good guide to the underlying geology of an area.  Differences in characteristics of rock (most notably pH) lead to different communities of plants and, to a lesser extent, animals, establishing on drystone walls.  The wall’s wildlife is influenced also by other factors such as orientation of the wall, the altitude at which it is found as well as the manner in which it is constructed.

A close look at any long established wall often reveals that what looks like bare rock is covered almost entirely in mosaics and patchworks of lichen.  The carboniferous limestone of the Craven area is a classic locality for ‘calcicolous’ lichens that love basic or alkaline substrates upon which to grow, but where there is acidic gritstone the lichen flora can be quite different in nature.

Nowell’s Limestone Moss is confined to seven sites all within the Yorkshire Dales, several of the sites being drystone walls in the vicinity of Pen-y-ghent.  It is a brownish-green moss that forms dense cushions. The moss is named after John Nowell, who first discovered the species in 1866.  It flowered that year, producing spores, but scientists had to wait over 130 years until 2003 to find fruiting bodies again.  It is legally protected, being listed in Schedule 8 of the Wildlife and Countryside Act 1981 (as amended).

Plants that like to grow in cracks and crevices or on wall tops include Biting Stonecrop, and Herb-robert while, the small fern Wall-rue, grows exclusively on calcareous substrates such as limestone or in lime mortar.

The interiors of drystone walls are often filled with smaller rock and rubble and the spaces within can shelter animals such as the tiny Wall Whorl Snail and Mountain Whorl Snail as well as nesting birds like Wrens and mammals such as voles, mice, Stoats or Weasels.

Nowell’s Limestone Moss