This type of damage can often be accidental. The most usual cases are caused by machines used to do maintainance within the cemetery.
Often ride-on lawnmowers will accidentally collide with a monument and cause damage such a breaking off part of it.
Another problem can be the use of strimmers or other cutting devices. If these come into contact with monuments made from soft stone then they can cause scratches to the surface. In time, these scratches will allow water ingress which will then lead to frost damage.
When grave monument are cleaned, these can also cause long term damage. If lichen is scrapped off the surface of a monument, tiny punture holes will remain. These will allow water ingress which will then lead to frost damage.
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