The oldest is the Midway limestone and clays in a narrow strip whose western limit is nearly parallel to the western boundary of the Selma chalk; it includes: the Clayton formation, characterized by the hard blue Turritella limestone (so named from the frequent fossil (Turritella mortoni); and Porters Creek (previously called Flatwoods) clay, which is grey, weathering white, and is occasionally overlain by grey fossiliferous sandstone.
Large areas are overlain with trachyte, basalt, obsidian, tuff and pumice.
In the north the plateau is overlain by red and purple unfossiliferous sandstones, capped near its edge by a cherty limestone also unfossiliferous but possibly of Lower Cretaceous age.
This system rests unconformably on the Witwatersrand series and is unconformably overlain by the Transvaal system.
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