EROSION – BY MOTHER NATURE OR THE HAND OF MAN?
We are all familiar with the damage that can be done to buildings in acts of vandalism, but the greatest enemy to buildings is erosion where wind and water gradually destroy the stonework.
St Mary's Episcopal Church provides a good demonstration of the powers of erosion. It was made from sandstone probably quarried from Granton in the north of Edinburgh, where the Duke of Buccleuch owned quarries. This reddish sandstone shows evidence of erosion sometimes so bad that the original stones have been replaced.
The main geological processes of erosion which affect the sandstone of this building involve water, from rain or snow. There is chemical erosion when this water reacts with minerals and chemicals in the stone and breaks it into fragments, and there is mechanical erosion when the water, either in cracks or absorbed by the stone, freezes, expands and causes the stone to split.
A good example of natural erosion lies on the north-west corner, in the path of the prevailing wind and rain. The original stones have been replaced by pale coloured sandstone.
But man, in this case well-intentioned, also hastens the process of erosion. Here, there is “Designed-in erosion” where the architect's design has unwittingly exposed some stone to more water than others, like underneath cornices or beside waterspouts.
Here too is “Preferential erosion” where repairs have been made using cement. Cement is non-porous, so throws off the rainwater onto the surrounding porous sandstone.
There are rumours that some of the erosion here is the result of the accidental mis-laying of the stone by the original builders. Blocks of masonry should be laid flat so that the bedding layers remain compressed. If a block is laid on its side these layers can peel off.
Much of the masonry has been decorated by stugging, a pattern of holes produced by masons using pointed chisels, a very time consuming process. The new pale grey stones have also been stugged, in a reasonably successful attempt to match the appearance of the original stugging.
Q1. Which geological process of erosion is responsible for the damage to this building?
Q2 . Can you match up the types of erosion listed below with the photographs, A-D, above? Please also state rough direction in which the affected stone (area marked with a?) is facing i.e. North , south etc.
Erosion by the prevailing wind and rain
Preferential erosion
Designed-in erosion
Mis-laid stone erosion
And can you identify other examples of these types of erosion, and send photos?
Q3. How does stonework stugged by hand differ from stonework stugged by machine? What kind of tool was needed for hand stugging?
Please send your answers via the message centre, where attachments can be added. You will then receive the answers and some more information about the geology and history of the church.
Now go ahead and log this cache, and a photo is always welcome. Remember, please do not add your answers to the cache page. You will be sent the correct answers and further a historical and geologically technical explanation of this earthcache.
This earthcache has been provided by the Lothian and Borders GeoConservation with permission of Buccleuch Properties in cooperation with the ranger service of Dalkeith Country Park http://www.dalkeithcountrypark.co.uk/