FIELD MEASUREMENT OF EROSION RATES: TIME-LAPSE MONITORING OF RAPID STONE FLAKING IN YORKSHIRE

The rapid deterioration of magnesian limestone buildings in the north of England has been a serious problem for more than one hundred years. While air quality in England has improved during this period, the rate of stone loss in these carved stone structures has not slowed. Thus far, conventional stone conservation treatments have not been successful in mitigating this decay, and large-scale stone replacement has been proposed to deal with the problem for buildings such as York Minster and the world heritage site of Fountains Abbey.

The use of a solar-powered, field time-lapse camera correlated with separately acquired environmental monitoring data, allowed the analysis of the pattern and rate of loss of stone from the surface of Howden Minster, an abandoned monastery in Yorkshire dating to 1380 AD. Acquiring a photograph every 1-3 hours over the course of a year allowed the stone damage to be correlated with local environmental conditions. Preliminary results indicate that loss is episodic rather than continuous and in some cases is related to unusual environmental conditions, such as high winds and condensation events. Damage was found also to be synchronous, with surface change (flaking, granular disintegration, and loss of flakes) occurring at the same time in different stone blocks. Crystallization pressures from magnesium sulfate phase transitions appear to be the main cause of the loss of stone surfaces. It should be noted that quantitative data on rates of surface loss are not available from most monuments.

Time-lapse methods permit the relatively inexpensive acquisition of this type of data, which is needed to aid conservation decision-making and the evaluation of interventions. Such tools should also prove useful to geomorphologists studying honeycomb weathering, the moving rocks on Death Valley's Racetrack Playa, and other phenomena that are otherwise difficult to study.