Antheia: Archiving due to being in a dry stone wall.
The guidelines for cache placement say this:
Caches are placed so that the surrounding environment, whether natural or human-made, is safe from intentional or unintentional harm. Property must not be damaged or altered to provide a hiding place, clue, or means of logging a find.
People searching for a cache in such walls have been known to take stones out. This can cause considerable damage to the structure. So to prevent damage to these walls we do not allow caches to be placed in, on or up against them. You may see more information in the UK Geocaching Policies Wiki.