*grumble* they'd be commonplace to you as well if you lived in New England! And I know there are other places in the states, and I am sure Canada where they occur a lot -

Not that our drystone walls can match the UK ones for age - though many in my home town are probably several hundred years old, the oldest up to 350 or so; though a good portion are probably less then fifty years old(zoning code iencourages their use in new landscaping)