In the very early years of the 19th century work began on the construction of Leith Docks. Before 1800 the port's trade had been carried out from the quaysides along the Water of Leith where it entered the North Sea, but as Leith became more industrialised in the later 18th century and demand for its goods grew it became clear that the existing arrangements for the docking and loading of trading vessels was hopelessly inadequate. John Rennie the Elder, a leading civil engineer, designed the new docks which were opened in 1806, but during their construction it became necessary to deal with a huge barnacle-encrusted glacial boulder on the shoreline in the vicinity of Leith Citadel. It was proposed by the foreman of works that this huge object be blown up, but it is recorded that the workmen charged with its destruction were not keen on taking on the job, due to the boulder's reputation as the home of a shellycoat.
In Scottish tradition a shellycoat was a sort of demon or spirit which haunted certain watery places. It wore a coat of seashells which produced a rustling sound as the demon walked, warning passers-by of its presence. Many of these shellycoats were said to be mischievous rather than violent, often amusing themselves at human expense by such pranks as calling for help, as if someone were in trouble in the water, and then moving elsewhere and calling for help again, thus leading people away from their intended path. The Leith shellycoat however had a more sinister reputation. At least one death was attributed to this demon, when he played football with a drunken visitor until the latter dropped dead through exhaustion! James Grant in Old and New Edinburgh (1880) describes the Leith Shellycoat as 'a sort of monster fiend, gigantic, but undefinable, who possessed powers almost infinite; who never undertook anything, no matter how great, which he failed to accomplish; his swiftness was that of a spirit, and he delighted in deeds of blood and devastation.' Such was the shellycoat's fearsome reputation that local youths often proved their courage by accepting the task of approaching the rock at sunset and running three times around it, all the while chanting:
"Shellycoat, Shellycoat, gang awa' hame,
I cry na' yer mercy, I fear na' yer name!"
This was no doubt followed by a very swift flight from the scene. Yet the reluctance of the dock builders to destroy the rock is an indication that respect for the shellycoat was not confined to children. The boulder was instead moved to the other side of Leith, to lie on the sands there, a task that could not have been easy given its size and weight.
Today the poor shellycoat's rock resides almost forgotten in a rather unglamorous location, the entrance to the local sewage plant. It appears that with its removal the reputation attached to the rock was exorcised, for it became simply known as the 'Penny Bap' thereafter, and local boys no longer dare each other to disturb its dreadful resident.