If you have seen these large concrete cubes on your walk along the trail or stared at them when you stopped at the picnic table, they are in fact World War Two Anti-Tank Blocks. Also known as Tank Traps, they were designed to slow-down a potential German invasion.
These specific ones were laid to defend the old railway line near Garth Isaf, Arthog, as railway lines offered easy transportation for infantry and tanks. There are six blocks, five are on the Estuary-side by the picnic table and a sixth set in the railway cutting on the opposite side.
There are other examples in the area, including Fairbourne Beach (see below) and beside Gwernan Lake near Dolgellau. These ones are set ‘corner to corner’, suggesting they were built at a slightly later period.