Before discussing the age of the Fluvio-marine series it will be well to describe its development in the field. The beds of the complex are throughout the district always found following on above the Qasr el Sagha beds, although their thickness varies considerably, as might be expected in a series of this nature. The most easterly locality to which the formation was mapped is the scarp 23 kilometres due north of Tamia, known as Elwat Hialla. Here the beds form a separate escarpment, consisting of only about 40 metres of sands and sandstone grit (sometimes silicified) with pieces of silicified wood: some of the beds of sandstone have a concretionary stem-like weathering. From this point these beds extend westwards far beyond the western end of the lake, always forming the highest escarpments of the Fayûm depression. A kilometre or two from our most easterly point the first basalt sheets are seen, and these, preserving the same level as far as can be observed, continue some 60 kilometres further west, to a point nearly due north of the western end of the Birket el Qurûn. The series, only 40 metres thick at the eastern end, gradually thickens as it is followed westward, until it reaches its maximum development in the cliffs of Jebel el Qatrani, north-west of the temple of Qasr el Sagha, where a thickness of some 210 metres is attained. Just 29 kilometres N.N.E. of Tamia (6 kilometres N.W. of the prominent western scarp of Elwat Hialla), a long hill offers a good section of these beds, which consist of a variegated group of green sands, red clays, coarse sandstones, red and yellow sand and sandstone, etc., capped by a band of hard impure yellowish limestone with numerous enclosed sand-grains (calcareous grit). Near the same place is an interbedded sheet of basalt, which is sometimes followed by another band of impure limestone and the latter by false-bedded sandstone. Huge logs of weathered-out silicified trees are seen strewn about. The following is a detailed section of the series, measured from a point 3½ kilometres W.N.W. of Elwat Hialla, and about 28 kilometres N.N.W. of Tamia, to the top of the escarpment 4 kilometres further north:— Undulating sandy, gravel-covered desert[66] stretching northwards. _Top of escarpment._ Metres. 1. False-bedded sandstones } } 2. Band of impure limestone } } 8 3. Interbedded basalt sheet } } 4. Sandstone } _(Section continued in hill ¾ kilometre further north-east)._ 5. Hard yellow limestone with enclosed sand-grains; cavities full of calcite 1 6. Greenish-white sand-rock 1 7. Hard reddish-brown stem-weathering sandstone 1½