For a bridge to stay together structurally it needs to be able to move. Rather than be so rigid that temperature changes or high winds or severe frost create pressure enough to cause it to crack, a bridge has 'expansion joints' built into it to accommodate the pressure allowing a certain amount of controlled movement.
A dry stone wall works the same way. It stays together like a basket. The stones in the wall create thousands of expansion joints. Each joint can move a fraction of inch if it needs to. The unsightly cracks one sees in a concrete or masonry block wall do not occur in a dry stone wall. It is engineered more like a bridge and will be less likely to become compromised structurally .
