The Carboniferous Limestone at Bullslaughter Bay hosts some of the most notable examples of deep weathering in the British Isles as well as two members of an enigmatic suite of breccias known as the Gash Breccias. The weathered limestone has been investigated thoroughly in order to identify the process responsible for the weathering. In this paper it is demonstrated that the weathering is isovolumetric but the weathering profile is not characterised by a vertical gradient and its depth suggests that meteoric waters did not contribute significantly to the weathering process. The weathered limestone has lost significant amounts of calcium and parts are virtually decalcified. It is seen that the dominant primary minerals of illite and quartz have been preserved while secondary clay minerals are generally absent. The weathered limestone cannot be a saprolite sensu stricto as it has been subjected to only restricted chemical processes. It is, therefore, interpreted as a “ghost-rock”. This type of weathering results from chemical dissolution by slow moving waters in the saturated zone. It is suggested that the weathering may have taken place during periods of emergence in the Carboniferous, at the same time as the cyclothem tops were exposed to subaerial modification, as evidenced by omission surfaces and palaeokarstic solution features. This is the first time that ghost-rock weathering has been reported from the British Isles.