The Clawdd-du, also known in historical records as the Black Dyke, Black Ditch or Clawthy,[1] is a mediaeval linear defensive earthwork or moat, constructed as protection for the faubourg of Overmonnow, on the opposite side of the River Monnow from the town and castle of Monmouth, Wales.

The suburb of Overmonnow, which in mediaeval times was an important area for iron working and the manufacture of coarse woollen material including Monmouth caps, would have been vulnerable to attacks from the Welsh at the time the defences were constructed.

[2] The ditch originally extended, roughly in a semi-circle, for some 600 yards (550 m) from the River Monnow opposite Chippenham Fields, westwards and then northwards to the vicinity of the later Drybridge House.

It is a Scheduled Ancient Monument, and the remains of the mediaeval bridge are a Grade II* listed building.

[6] In 2010, local archaeologists objected to Monmouthshire County Council because a trench had been dug along the site without proper consent.