Brian Mulcahy sent me this photo recently.
While driving back and forth from work each day, he had been admiring, from a distance at least, what had to be a newish dry stone wall built with quarried sandstone.
Then one day seeing a section of it had mysteriously been 'taken down' or worse 'come apart' he stopped the car and walked over to inspect. The dry stone wall appeared to be nothing but a sack of gravel loosely held in place by a sandstone veneer. It was bound to fall apart.
Apart from all the other structural mistakes this wall exhibits, having the insides of a wall filled with gravel material (instead of properly fit individually-placed appropriately-sized pieces of stone rubble) almost always leads to bulging and collapses like this one, usually within the first year.