_Cleopatra bulimoides_. _Hydrobia stagnalis_. _Valvata nilotica_. _Melania tuberculata_. _Planorbis Ehrenbergi_. _P. marginatus_ var. _subangulata_. Blanckenhorn[99] has pointed out that in this fauna _Hydrobia stagnalis_, as a typical brackish water form is of special interest. The species appears to have established itself in the Birket el Qurûn in modern times, as it has not been found in the youngest alluvium of the lake, nor is it known in the modern Nile fauna. [Sidenote: Modern Deposits: Blown Sand and Erosion.] Except for the gradual accumulation of silt over the bed of the Birket el Qurûn—sand and clay carried in by the wind and the fine sediment borne by the feeder canals—the only modern deposits of any importance are those of blown sand. The extensive arenaceous deposits of younger Tertiary age, forming the greater part of the continent from the latitude of the Fayûm to the Mediterranean shores, yield as a result of the action of denudation a constant and abundant supply of the raw material. The sand carried southwards by the prevailing winds accumulates as dunes in the lowest parts of the depressions, on the slopes of cliffs, and in all the less exposed localities. Wind swept areas remain free or are only gradually encroached on by slowly growing linear dunes originating in the wind-shadow of some protecting hill or ridge. An unique example of such a dune is to be seen at the south end of the well-marked ridge a few kilometres east of Gar el Gehannem (Plate XV). The main accumulations of blown sand are in the southern part of the Fayûm; large areas of the floors of Wadis Rayan and Moêla are covered with dunes, while in their immediate neighbourhood the material has accumulated to such an extent as to blot out entire cliffs and valleys; immediately to the west of Gharaq a considerable area is covered with small but steep dunes; and finally must be mentioned the great linear belt of sand, known as the Ghart el Khanashat, which starting from a point about midway between the Wadi Natrûn and Mogara comes to an abrupt termination some 24 kilometres before gaining the northern escarpment of the Fayûm depression (see page 23). As might be expected in an area like the Fayûm, where sedimentary rocks of every type are met with, and where the wind never wants for a sufficient supply of the necessary sand, superficial erosion is everywhere well marked. We do not propose to study here the action of wind-borne sand and it will be sufficient to mention two localities where the effects are best seen; one is in the neighbourhood of Garat el Esh, where the most remarkable scoring and grooving is to be seen on the two beds of limestone capping the upper and lower cliffs of the Middle Eocene; the other is the Zeuglodon Valley, and here the sculpturing of the sandstone of the Birket el Qurûn series is of the finest and most unique description. [Illustration: PLATE XVI. THE BIRKET EL QURUN NEAR THE WESTERN END.]