The geology of Western Sahara includes rock units dating back to the Archean more than two billion years old, although deposits of phosphorus formed in the Mesozoic and Cenozoic have helped to prompt the current[when?]
[1] The Reguibat Shield is part of the West African Craton, a large area of stable continental crust left over from the early formation of continents.
Extensive sedimentation in the Devonian preserved brachiopod, bryozoan and coral fossils in sequences of oolitic limestone, sandstone, siltstone and clay up to one kilometer thick.
[2] Parts of southern and north-central Western Sahara have multilayer unconsolidated sand and gravel aquifers, from the Cenozoic, subdivided by impermeable silt and clay layers.
In the south and east, fractured Precambrian basement rock stores some water, but with very limited recharge due to exceedingly low rainfall totals.