The Nile Valley, the Fayûm and Wadi Raiân depressions were the ultimate result of this action, the formation of the Nile Valley being completed by the flow of water. At first the upper reaches of the Nile Valley held its waters at a high level by barriers of rock, but in process of time these barriers were cut through, the bed scoured out by the constant flow of water, and the water surface lowered beyond its present levels, to be again gradually raised to the levels of to-day. The lower reaches of the Nile Valley were probably at first occupied by the sea, until the yearly deposit of the floods formed the Delta, and pushed the land thus formed further and further out, forcing the sea to retire. As the surface of the Delta became raised and prolonged by successive annual deposits, the bed and water-surface of the Nile also would have risen with it, until the levels at which the Nile flowed in its lower reaches became those of the present day. At a point in the hills dividing the Nile Valley and the Fayûm, about 10 kilometres south of Lahûn, near Sidment-el-Gebel, Dr. Schweinfurth found “the indubitable witnesses of a Pliocene sea” preserved in the form of oysters (_Ostrea cucullata_ and _Pecten_) in the white sand at about R.L. 60 to 70. (See map and Plate XXI.) The Pliocene sea, he maintains, intruded up the Nile Valley and extended on both sides of it as far as the contours of 60 to 70 metres above sea-level allowed. The place, where the oysters in the white sand were found, is situated in a flat depression on the plateau of the narrowest part of the hills separating the Nile Valley and the Fayûm. The Pliocene sea flowed here from one depression into the other, and would have succeeded in scouring away the barrier between the two depressions, if it had not been interrupted by a later upheaval or a withdrawal of the sea. In a similar manner there seems to have been made from Lahûn to Hawârah, the present communication between the Nile Valley and the Fayûm by which the Bahr Yûsuf entered to form Lake Mœris, either as an old Nile-arm or as an artificial branch of the natural arm. Besides this passage, the desert tract on the north of it offers, as breaks in the higher ridge, several depressions, which must have been accessible to the Pliocene sea. The present railway line to the Fayûm crosses the hills at one of these depressions. On account of the regularity of the limestone strata in the Fayûm and Wadi Raiân, a violent upheaval cannot be supposed to have been the cause that produced these two depressions, and it is more likely that they are the results of erosion and scour.