This map (After Malim et al 1996) is a map of South Cambridgeshire and shows the five Cambridgeshire dykes - Bran (or Heydon) Ditch, Brent (or Pampisford) Ditch, the northern Fleam Dyke (or High Ditch), Fleam (or Balsham Dyke) and Devil's Dyke, plus Roman roads.
The nearby village of Fen Ditton is situated in Flendish Hundred and is mentioned in documents before 991 AD and the name Fen Ditton itself means ‘farm by the ditch’ in Anglo-Saxon. A Medieval field at the eastern end of Fen Ditton was called High Ditch Field earlier than the 19th century and High Ditch itself is shown on a map of 1731-2 (County Record Office CRO TR626/P1), lying south of the road from Fen Ditton to Quy which cut High Ditch Field in two. On some Ordnance Survey maps the ditch is shown on the north side of High Ditch Road, although there is little visible today and we know that High Ditch Road is being called that name in documents by 1821. The map below is an old OS map dating from the 1950's (pre-A14!)
Fox noted in 1923 (Fox, C. & Palmer, W. M. 1923 'Excavations in the Cambridgeshire Dykes II: The Fleam Dyke' Proceedings of the Cambridge Antiquarian Society XXIV, 28-53 ) that the Fen Ditton section of the Fleam Dyke was most probably just a local defensive earthwork when it was first constructed, and he suggests that the ditch was only later incorporated into the larger system that includes the Fulbourn section of the Dyke. You can see the sections of the Fleam Dyke on the first map above.
High Ditch (like all the Cambridgeshire Dykes) may then have had various purposes including being a political boundary restricting movement along the good arable land between the Fens to the North West and the wooded hills to the South East and a military defence by the incoming Anglo-Saxon settlers against British attacks from the South West.
As well as Fox's article, a more recent and very informative article on the Cambridgeshire Dykes and archaeological excavations associated with them is Malim, T. et al 1996 'New Evidence on the Cambridgeshire Dykes and Worsted Street Roman Road' in Proceedings of the Cambridge Antiquarian Society LXXXV, 27-122.