The Bahariya Oasis is an oval shaped depression trending in a NE-SW direction. Within the depression, the Cretaceous rocks outcrop at its base as well as at the base of the conical hills and the scarp. The Cretaceous rock succession (Sabaya, Bahariya, El Heiz and El Hefhuf Formations) comprises fluvialite to fluviomarine clastics of sandstone, claystone and shale. The Sabaya Formation of Lower Cenomanian age is coverd by Bahariya Formation which also belongs to the Lower Cenomanian. The Bahariya Formation is unconformably overlain by the Upper Cenomanian fluviomarine marly shale, sandy dolomitic limestone and calcareous sandstone of El Heiz Formation as well as by the Campanian cherty cavernous dolostone, crossbedded sandstone and phosphatic limestone of El Hafhuf Formation.
The succeeding Maeastrichtian Khoman Formation is mainly represented by chalk and chalkly limestone. It overlies conformably El Hafhuf Formation on both sides of the depression and extends southward with increasing thickness. The Eocene rocks are represented by the Naqab Formation which belongs to the Middle Eocene and comprises gray and pink limestone and by the Qalamoun Formation which belongs to the Lower Eocene. The Qalamoun Formation is deposited on the Bahariya Formation and El Hafhuf Formation to the North of Bahariya depression. The succeeding Oligocene Qatrani Formation covers Bahariya Formation at the top of the conical hills besides occurring as small outcrops within the depression. This formation consists of quartizitic sandstone, quarzite, shale and silt.

At the northern part of Gebel El Hefhuf, Oligo-Miocene basaltic and doleritic extrusions are recorded. The types of volcanic rocks in this region can be divided into three main varieties, mainly an amygdaloidal basalt which is the oldest extruded lava followed by the intruded dolerite. Later another period of volcanicity took place and the olivine basalt porphyry was extruded covering the amygdaloidal basalt in Gebel El Hefhuf and the small basalt hill nearby, and helped to cover and preserve in these two hills the layer of amygdaloidal basalt below it, while in Gebel Maysarah and Mandisha where this basalt porphyry was not represented caused the lower layer of amygdaloidal basalt to be also absent and probably leached.
As to the age of these volcanic rocks there is no doubt that it is post-cretaceous. The Eocene being not represented in this part of the oasis leads the writer to conclude that the volcanic rocks are probably of Oligocene age specially that this period of Tertiary volcanicity was recorded in Egypt. The eruption was at several steps and took a long period. The variation in types of rock and alteration indicate that the formation of the volcanic rocks in this area took place at several times and more or less during long periods of volcanicity.
Columnar Oligocene flood basalt sheets cover the Cenomanian Bahariya Formation at Gebel Mandisha area in the Bahariya oasis depression. The Mandisha basalts are located with the position of 28� 54' E and 28�22' N, nearby the iron ore mines in the eastern direction. The basaltic intrusions took place during Oligocene, when the Gulf of Suez rift began to open. As noted earlier, hydrovolcanic solutions associated with this subvolcanic activity caused intensive mineralization and iron precipitation in parts of the depression. Iron forms as a replacement to Eocene limestone where open cast quarries are located in several places. Mostly known are the mines of El Harra area and El Gedida area at the northern edge of the depression.