Geology Reference
In-Depth Information
(a)
(b)
Figure 9.8
(a) Diagram showing hórreo or maize-storage house on stilts, and the weathering of part of
granite stilt not colonised by lichens. (b) Galician hórreo from Laxe, A Coruña, northwestern Spain.
important, for overhanging sidewalls are developed in areas, such as Dartmoor, southwestern
England, and the Yosemite region of the Sierra Nevada, California, where there is no detectable sur-
face induration (but where there are lichen colonies).
The role of lichens is difficult to assess (Scott, 1967; Fry, 1926). It has been shown that lichens
are agents of weathering. Their hyphae penetrate the rock and assist in physical disintegration.
They also facilitate water entry and in addition extract selected minerals for their own use. On the
other hand, and without denying that lichens cause weathering, there is observational evidence that
they protect rock surfaces. They not only stabilise the surface in an absolute sense but the rate of
weathering is greater on adjacent exposed or non-colonised surfaces. Forexample, in many parts of
northern Spain small houses on stilts called hórreos are constructed for the storage of corn (maize)
heads. The house sits on a platform which is much wider than the spacing of the stilts, the over-
hang preventing rodents climbing up to the house and the maize (Figs 9.8a and b). Such an hórreo
was built of the local stone, granite, at Louro, A Magdalena, near the Ría de Muros, in southern
Galicia in 1950. Within 50 years lichens had colonised those (lower) parts of the stilts exposed to
rain from the west, but the upper parts, in the shade of the platform, were not colonised. The
exposed granite clearly suffered disintegration, whereas the areas covered by lichens remained
