That's interesting. I wonder if another factor is a wide variation in the level of skill used to build the walls. It's easy for a complete beginner to start bui

That's interesting. I wonder if another factor is a wide variation in the level of skill used to build the walls. It's easy for a complete beginner to start building dry stone walls based on a limited amount of information - and using only materials found on site. Those walls might last the lifetime of the builder, but are less likely to hold up to stress over the ages.