A dry stone wall uses no mortar. Each stone is chosen for its shape and placed so that gravity and friction do the work of holding things together. A good waller can read a pile of rubble the way a typesetter reads a case of letters — finding the right piece for the right gap without conscious calculation.
The walls that cross the Yorkshire Dales have stood for centuries, built by hands we will never know, marking boundaries that no longer matter for sheep that are long dead. They persist anyway. A well-built dry stone wall actually improves with age; frost and rain settle the stones more tightly into place.
I spent a week learning to build one in Wensleydale. My section is identifiable by its slight wobble. I am told this will correct itself in about fifty years.