Seahenge was originally constructed on a salt marsh, and over the centuries the area became a freshwater wetland, as an offshore barrier grew up, preventing sea water from getting access to the area around the circle. This in turn allowed alder trees to grow in the area, which eventually created a layer of peat above the mudflats. With rising sea levels in later millennia, the sea advanced, and eventually sand began to cover the peat. Through this process, Seahenge eventually found itself from once being inland to being on the beach, where it was revealed by the eroding away of the sand and peat by the late 20th century, four thousand years since its original construction.