The Great Fourstone is one of Lancaster's more obscure curiosities: a large glacial erratic — a boulder carried here by ice sheets during the last glacial maximum — that sits in the city with a stone staircase carved into one side so that people can climb to the top. The carving is not ancient; it was added in the 19th century when the boulder became a curiosity worth visiting in its own right rather than simply an obstacle.
The combination of ancient geological provenance (the rock was deposited here roughly 12,000 years ago) and Victorian civic improvement is very Lancaster. It is a short detour from the main sights — worth a look if you are wandering the streets and enjoy geological or historical curiosities. Not a destination in itself, but the kind of thing that makes a city feel layered.