Antonine Wall near Bar Hill Roman Fort, near Cumbernauld.
The high tide mark of Roman expansion northwards into Britain, the turf wall is less famous than its older stone-built brother. Clearly a wall too far, it was soon abandoned and the frontier re-established at Hadrian's wall further South.
This cameraphone shot doesn't give much impression of the scale, but after 1800 years it is still enormously impressive. Or impressively enormous.