The Purana Group coincides with the Proterozoic Period of the earth’s geological history (600 million years ago). It is separated from the underlying Archaean Group by an unconformity called the Eparchaean unconformity.
The closing of the Dharwar period witnessed intense eardi-move- ments which folded the Dharwar sediments into mountain ranges (the Aravallis being die most prominent). During a long period of time following afterward die Dharwar landscape was eroded down to the base level stage.While this process was going on new basins were formed due to local warping and subsidence wherein the eroded material from the Dharwar area was accumulated.
These new rocks belong to die Purana Group which largely consists of sedimentary, metamorphic and mixed rock-types. Their lower strata contain iron mixed jasper, while upper layers exhibit deposits of shale, limestone and sandstone. These also have intrusions of lava dykes. The Purana Group consists of two rock systems- the Cuddapah and die Vindhyan (lower).