Part of the ditch of Clovelly Dykes hillfort, Devon.
This site is a mystery - it seems to be an Iron Age hillfort but it is not on a hill making it hard to defend. It is huge (over 8 hectares) but no clear dating evidence has been found (it has only had minimal trial excavation) - what is certain is that somebody put a lot of work into building it.
See :
http://www.ndas.org.uk/dykes.html