Painting by an unknown artist, depicting Treath Mawr or The Cob, Porthmadog, in the 19th century.
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 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.