A dry stone wall is a purposeful pile of rocks, held in place by friction and gravity rather than mortar. It’s one of the oldest building methods known to mankind, used over millennia to construct buildings, wind breaks, and dykes, to mark borders and create monuments, and to keep livestock penned in and nuisance animals hemmed out. In rural Japan, where I live, stone retaining walls shape a country that’s 80 percent mountainous into livable and farmable land. To get into town from my home, I walk past century-old stone walls. The rocks are shaded velvety green by moss, and plumes of ferns spring from crevices with starry pink flowers hovering above. These walls carve out spaces for roads and houses, and define irregular terraced rice paddies farther up in the mountains. But closer to town, stone walls give way to walls made of concrete, which has largely replaced stone as the defining material of Japan’s rural infrastructure. Is that because concrete construction is cheaper, stronger, or faster? Not really. Reviving dry stone walling would …