The Cob, (partly in Penrhyndeudraeth)

The Cob is a substantial rubble-built embankment that stretches approximately 1.5 kilometers long, consisting of a lower section for road traffic and a higher section for railway use. The higher part of the embankment stands about 6 meters tall and accommodates a single-track railway alongside a flanking footpath. The later, much lower embankment built in 1836 features a narrow double carriageway.

On the seaward side, the embankment is heavily buttressed with boulders that slope gradually down to the water, while the landward side has less substantial buttressing, primarily consisting of a grassed bank. The parapets are made of rubble slate-stone and range from approximately 0.5 meters to 1 meter high, except on the seaward side where they are constructed of concrete. The parapet separating the roadway from the cycle track is made from stone sourced from Dyffryn Nantlle, replacing an earlier wall made from stone from Blaenau Ffestiniog.

Evidence of extensive repairs is visible in the masonry breaks and changes in the coping along the wall that divides the higher and lower sections. At the Porthmadog end, the parapet of the higher embankment ends opposite No 6 Tros Y Bont and next to the curve leading into the railway station. Steps leading down to the shore and the road are located on both sides of the higher embankment at the eastern end. Beyond this point, the railway tracks either curve northeast or continue towards the Boston Lodge Workshops. Beneath the curve, there is a semicircular arched recess that is now blocked but was originally a passage beneath the railway, featuring a portal-like entrance with coursed masonry and voussoirs. The Cob concludes at the Boston Lodge end with gated steps situated between the railway line and the road.