The Merensky Reef is not a specific lithological horizon but it is a zone of platinum-group element-bearing sulphide mineralization which straddles the unconformity surface at the base of the Merensky Cyclic Unit. The Merensky unconformity surface forms a contact with different footwall lithologies. The nature of the unconformity is such that it forms cross-cutting relationships with the readily thermally erodable leucocratic rocks and conformable relationships with the thermally resistant melanocratic rocks. The nature of this unconformity is often expressed as potholing. The Merensky magma reacted to varying degrees with different footwall units, superimposing platinum group element (PGE) mineralization into the footwall and onto the pre-existing PGE mineralization. On Northam there is no constant metal balance or constant grade within the Merensky Reef.