The Cambro-Ordovician Saq Sandstone, over 600m thick, is a quartz-arenite and rests unconformably on the Arabian Shield from the Al-Qassim region in the South to the Jordan Border, Quartz generally comprises more than 95% of the sandstone and 91% of the quartz is monocrystalline quartz. Both chemistry and petrography studies indicate this Formation to be very mature sandstone. The Saq Sandstone is abundantly cross-stratified and contains marine fossils. It is considered to have formed on an open marine tidal shelf. The cross-strata are sometimes seen to form into major sand bars, and beach deposits occur within the sequence. The palaeoflow is unimodal, and is the result of asymmetrical tides operation on a sloping shelf. The palaeoflow, which is to the ME, is therefore seen as the main direction of sediment dispersal and is the direction in which the Cambrian sandstones in this whole area are replaced by mudstones. The Saq Sandstone was derived from a pre-existing sandstone since it contains detrital chert. In the process of becoming mature, there are probably many cycles of reworking before final deposition. There is some 50 Million years difference between the last recorded thermal event affecting the Arabian Shield and the deposition of this sandstone. This suggests that the development of a craton can occur in less than 50 Million years
