Two forms of dedolomitization. surface and near-surface, are found in the dolomitic Carboniferous rocks of the Middle Limestone Group of the northeast coast of Northumberland, England. This work is primarily concerned with the surface form of dedolomitization which is taking place in the Recent and sub-Recent, rusty-looking weathering crusts of the ferroan (iron-rich) dolostones. The rusty colour is caused by the presence of iron hydroxides in the weathering crusts. These iron hydroxides and the associated dedolomites are genetically related. The susceptibility of ferroan dolomites to dedolomitization is apparently caused by their metastability in the surface environment. Circulating sea or fresh water along permeable zones in the carbonates exposed at or near the present erosional surface is responsible for the oxidation and hydration of the ferrous iron content of the metastable ferroan dolomites as well as the dedolomitization of these dolomites. The dedolomitization mechanism has Been found to be a process of great potential in converting highly impermeable and impervious dolomitic rocks into a porous and permeable state. The detection of such a phenomenon in carbonate rocks at depth may, therefore, not only indicate an unconformity, but may help in the prediction of potential oil or gas reservoirs.