Is it possible to take an interesting photo of a wall? Mariana Cook certainly thinks so, and has published a book of dry stone walls around the world - Stone Walls: Personal Boundaries.
Her attention was drawn to the walls when one collapsed at the back of her garden in Martha's Vineyard, and she found 56 cows leaving hoofprints in her newly sown lawn.
"I started looking more closely at the wall, noticed the lichen, how beautiful it was, and... the tumbled stones," she says. She photographed it and sold the prints to pay for the wall's repair.
A portrait photographer by trade, she mentioned the project to a British archaeologist she was photographing. He took her to Derbyshire, and showed her how the history of each wall could be read in the stones.
The archaeologist persuaded her to visit the Aran Islands in Ireland, where she took her favourite photographs in the book, of dry stone walls as much as 2,000 years old.
"Each farmer built his wall to surround his field. They're using the same material, the same rock, and yet the walls differ one from the other... it's a kind of vernacular art."
Most of her photos concentrate on walls built by one or two people, not walls that were built for "purposes of power", but the Inca walls around Cuzco in Peru forced their way into the book.
"The way they put those boulders, stones together, is so remarkable. I just decided 'I'm an artist. I can do what I want,'" she says.
"There is something captivating about all of these walls," she says. "Part of it is that there was someone there before you. You know that they were there, and you can see what they did and can read what they did."
"I can't really think of anything else that has existed, and continues to be built for such a long time," she says. "I'd like to think of one, because I'd like to go photograph one."
What are these?