Stone Age technology for tectonics.

Taking a clue from Neolithic civilizations, English geophysicists are using ancient designs to create an extremely stable surveying site intended to last for decades, if not millennia. The mixture of Stone Age and Space Age methods will enable them to track the subtle movements of the continents with extreme precision, says Geoffrey Blewitt of the University of Newcastle in England.

Blewitt and his colleagues monitor crustal motion using a receiver that picks up signals from the Global Positioning System (GPS), a constellation of 24 satellites orbiting Earth. To ensure the quality of these measurements, they need to place the receiver's antenna on a solid benchmark whose position will not shift over many decades. Their study site in northeast England, however, has little exposed bedrock.

Many geodesists get around that problem by putting antennas on top of buried concrete pillars, but Blewitt rejected this approach because concrete can deform and shrink over the years. Instead, the Newcastle team took inspiration from the standing stones known as menhirs that dot the landscape of western Europe. Some of these monuments have endured for 4,000 years, a testament to their stability.

The scientists carved a replica of a menhir out of a single piece of carbonate rock, a flat-topped pyramid measuring 2.4 meters high and 1.6 meters across at its base. To ensure that the modern menhir does not move, the researchers excavated a hole in a field and cemented the base of the monument into the natural bedrock. The top of the menhir is flush with the ground surface. "We basically built our own rock outcrop," says Blewitt. "The only way it would move is if someone digs it up. But it's going to be very difficult to take it out. It weighs 4.5 tons." By making measurements every 30 seconds, they hope to monitor its position relative to other sites with an error of less than 1 millimeter.