Warm tropical Carboniferous seas covered much of Swaledale and Northern England for many millions of years. These shallow seas were full of life, and as these creatures died, the calcium carbonate from their shells and the sea water was deposited on the seabed to form the horizontal layers of limestones. Occasional influxes of sediment off the land and upstanding blocks introduced sand, silt and mud clouding the waters and eventually settling on the sea floor. The Main Limestone (or Great Limestone) of the Alston Formation forms one of the prominent units in Upper Swaledale and along with other limestones (e.g. the Iron Post, Five Yard, Three Yard and Four Fathom limestones) contains features that tell us about the Carboniferous seas, as well as later processes which sculpted the rock.
Where the limestone is highly fossiliferous (e.g. crinoidal), some of the beds can be hard enough to be used for decorative stone purposes. Thick beds of crystalline sparry crinoidal limestone were also quarried to provide large strong blocks needed in the construction of the huge pillars for the viaducts of the new railways which crossed areas of North Yorkshire. However, the limestone beds commonly change character and thickness rapidly across the region.
A particular fossiliferous limestone is the Underset Limestone (also known as Four Fathom Limestone). Loose blocks and boulders of this limestone are commonly found in many of the becks and the River Swale. The coarsely fossiliferous limestone beds from this unit were quarried for building stone at Melsonby and were marketed as ‘Swaledale Fossil Limestone’. This is a very typical limestone depicting the deposition of the limestone in warm tropical seas in Swaledale.