Sweet Track is the earliest known trackway—constructed footpath—in northern Europe. It was built, according to tree ring analysis of the wood, in the winter or early spring of 3807 or 3806 BC: this date supports earlier radiocarbon dates of the early 4th millennium BC. The trackway ran about 1800 meters across the peat marsh called Somerset Levels near the town of Glastonbury in England.
The walking surface at Sweet Track consisted of a line of flat wooden planks which were raised above the ground and water surface of the fen by a set of crossed poles, carefully pegged into the ground.
The Sweet Track is one of several trackways built during the Neolithic and Iron Age that are preserved in the boggy bottoms of the Somerset Levels, such as Meare Heath and Abbots Way. It is the oldest of all those discovered to date, and the most sophisticated. Inerestingly, it was also quite emphemeral, used only for about a decade before being engulfed by the fen.
Excavations in the Somerset Levels were conducted in the 1970s by John Coles of Cambridge University.
Beckett, S. C. and F. A. Hibbert 1979 Vegetational Change and the Influence of Prehistoric Man in the Somerset Levels. New Phytologist 83(2):577-600. Free to download
Hillam, J., et al. Dendrochronology of the English Neolithic. Antiquity 64 (243):210–220.