Twenty-year weathering remeasurements at St Paul's Cathedral, London

Microerosion meter (MEM) measurements of the surface height of the balustrade of St Paul's Cathedral, London, have been repeated in the year 2000 following earlier measurements in 1980, 1981, 1985 and 1990. Methodological sources of error mean that while the measurements were made to 0.0001 mm, the data are reliable to two decimal places. There was a reduction in the mean erosion rate on horizontal sites from 0.045 mm a^-1 in the period 1980-1990 to 0.025 mm a^-1 in 1990-2000. Decreases in atmospheric SO₂ levels from 20-25 ppb in 1980-1982 to around 10 ppb in 1990-2000, offer a causal explanation. The surface topography evolved more erratically in 1990-2000 than before, with much, but not all, of the more microelevated areas showing greater, and often more variable erosion. There are also indications of less erosion and more surface rises in low-lying microareas on horizontal sites which is interpreted as possible deposition and/or microfloral growth in wetter depressions, the pattern being largely absent on a well drained vertical site.