No walk in the New England woods is complete without a scramble over a ragged, lichen-covered stone wall. It’s estimated that 250,000 miles of walls once wound through the birch stands and cleared meadows of the Northeast, the majority of them likely built by farmers between 1775 and 1825. Though constructed fairly recently, stone walls seem a permanent part of our landscape. “[They] give us a clock by which we can judge the passage of almost unimaginable time,” writes University of Connecticut geologist Robert Thorson in Stone by Stone: The Magnificent History of New England’s Stone Walls.

In the 1700s, farmers found hard-rock fieldstones strewn over New England, left there by the grinding departure more than 20,000 years earlier of the Laurentide ice sheet. Farmers clearing their fields encountered the stones en masse, pushing up from the soil in sizes handy for heaving a short distance. The walls the farmers created served to mark property boundaries and protect livestock and fields, and piling the stones this way made the work of plow horses easier. Then, with the birth of the Erie Canal, railroads, and the industrial age, farming grew less vital, writes Thorson, and the heyday of the stone wall began its wane. Untold numbers of walls were disassembled for bridges and roads. Still, many stone walls survive today because forests grew back over the thousands of acres that had been cleared, leaving the walls to rest in peace. And there they have sat for decades, draped in gray-green lichen and inspiring poets, painters, and...scientists.

Scientists like Thorson use the lichen decorating stone walls to determine the age of those walls. This technique, called “lichenometry,” was discovered in the 1930s. Here’s how it works: New England rocks generally host three types of lichen: foliose, crustoce, and fruticose. Knowing that crustoce, for instance, grows at a rate of 1 millimeter per year, scientists will measure that lichen from the center to the outside edge and compute the age of the wall.

Given perfect conditions—i.e., the lichen began growing the year the wall was built, pollution has been minimal, and the rocks were lichen-free to begin with—the information gleaned might be accurate. But, considering how imperfect conditions have probably been—that it took some time before the lichen began growing, the rainfall has carried pollutants, and some of the rocks may have held a smattering of lichen when the wall was constructed—lichenometry is a somewhat loose science at best.

Another New England icon is helping perfect the practice, though. Thanks to the inscriptions on granite gravestones, the dates they were placed in the earth are pretty indisputable. Working with aging headstones has proven useful in testing the accuracy of the lichen-dating method.