Page 180 - Fundamentals of Geomorphology

WEATHERING AND RELATED LANDFORMS 163

Plate 6.4 Weathered balustrade on the Ashmolean Museum, Oxford, England. The balustrade has now been cleaned.
(Photograph by Heather A. Viles)

10 microns per year in an urban industrial site (Attewell Salt weathering is playing havoc with buildings of
and Taylor 1988). ethnic, religious, and cultural value in some parts of the
In the last few decades, concern has been voiced over world. In the towns of Khiva, Bukhara, and Samarkand,
the economic and cultural costs of historic buildings which lie in the centre of Uzbekistan’s irrigated cotton
being attacked by pollutants in cities (Plate 6.4). Geo- belt, prime examples of Islamic architecture – including
morphologists can advise such bodies as the Cathedrals mausolea, minarets, mosques, and madrasses – are being
Fabric Commission in an informed way by study- ruined by capillary rise, a rising water table resulting
ing urban weathering forms, measuring weathering from over-irrigation, and an increase in the salinity of the
rates, and establishing the connections between the two groundwater (Cooke 1994). The solution to these prob-
(e.g. Inkpen et al. 1994). The case of the Parthenon, lems is that the capillary fringe and the salts connected
Athens, was mentioned at the start of the chapter. with it must be removed from the buildings, which might
St Paul’s Cathedral in London, England, which is built be achieved by more effective water management (e.g.
of Portland limestone, is also being damaged by weath- the installation of effective pumping wells) and the con-
ering (Plate 6.5). It has suffered considerable attack by struction of damp-proof courses in selected buildings to
weathering over the past few hundred years. Portland prevent capillary rise.
limestone is a bright white colour. Before recent cleaning, Weathering plays an important role in releasing trace
St Paul’s was a sooty black. Acid rainwaters have etched elementsfromrocksandsoil,someofwhicharebeneficial
out hollows where it runs across the building’s surface. to humans and some injurious, usually depending on the
Along these channels, bulbous gypsum precipitates have concentrations involved in both cases. It is therefore rele-
formed beneath anvils and gargoyles, and acids, partic- vant to geomedicine, a subject that considers the effects
ularly sulphuric acid, in rainwater have reacted with the of trace elements or compounds in very small amounts –
limestone. About 0.62 microns of the limestone surface usually in the range of 10 to 100 parts per million
is lost each year, which represents a cumulative loss of (ppm) or less – on human health. For example, iodine is
1.5 cm since St Paul’s was built (Sharp et al. 1982). essential to the proper functioning of the thyroid gland.