The Cob is a 1.4 kilometre long causeway crossing Traeth Mawr; the original embankment was conceived by William Maddocks and constructed between 1808 and 1811. It was supplemented by a lower embankment in 1836 on the inland side which carried the turnpike road, whilst the narrow gauge Ffestiniog Railway (NPRN 34660) line was laid on the original embankment. The seaward side of the original causeway is rock armoured; the retaining wall between the two embankments is constructed of random laid slate, dry laid in long blocks at right angles to the slope. A similar wall supports the inland side of the lower embankment, which is partially protected by a rock apron at its base.

RCAHMW, 22 May 2008.