The geological oddity that is Sarn Wallog

Cardigan Bay, Ceredigion Made up of rounded cobbles graded in size, this 'causeway' looks beguilingly like the work of our ancestors

The amount of ice on the narrow footpath came as a surprise. Hidden from the sun on the south side of the valley, it had probably accumulated over a number of days - along with the layered, crusted frost on the nearby vegetation. Few people seemed to have walked this stretch of the Wales coast path recently; a fox crossing the track ahead and a briefly perched buzzard both seemed shocked to see me.

As I reached the footbridge above the beach at Wallog, I folded away my heavily used Ordnance Survey map and consulted its nautical equivalent. While OS maps provide only scant information about the space beyond the coast, admiralty charts give detailed data on the underwater landscape - in this case the geological oddity that is Sarn Wallog.