And the lichens keep eating

Stone wall at what had been Robert Frost's farm in Derry, N.H., which he describes in his famous poem "Mending Wall"

"Many New England stone fences built between 1700 and 1875 were laid by gangs of workers who piled stone at the rate of so much a rod.  Edwin Way Teale says that in the latter years of the 19th Century, before economic and social developments began obliterating some of the walls, there were a hundred thousand miles of stone fences in New England.  Even today, for many of them, the only change has been the size of the lichens, those delicate rock-eating algae that can live nine hundred years."

William Least Heat Moon, in Blue Highways