The most important fold is a well-marked syncline beginning at the west scarp in lat. 28° 7′ N. and running across the depression in a direction 30° north of east until it strikes the eastern escarpment in lat. 28° 23′. This synclinal fold is remarkably sharp, the width being generally only a few hundred metres, but the dips so strong as to bring the limestone beds nearly vertical in places. Along the line of fold we find the whole of the series 6 and 7 brought down to the level of the floor of the oasis, but on either side the beds so quickly regain their horizontal position that no trace of the folding is to be seen (Plate VII). The White Chalk in these folds is always altered by pressure into a hard crystalline limestone, as are also the limestones of the Cenomanian below. The fold is not actually continuous across the area, but is in reality made up of a number of long narrow ellipsoidal depressions, or cup-shaped hills, in which the end-dips are always less strong than those of the sides. At its most southerly point, where it runs into the scarp 14 kilometres north-west of Ain el Haiss, the two series 6 and 7 are found dipping northwards at angles up to 40° (and at one spot 78°). On the other side of the fold the same beds dip southwards at only 5°, and the appearance of the fold at this point suggests the existence of an accompanying fault (fig. 1). [Illustration: Fig. 1.—Section across Syncline, 12½ kilometres N.W. of Ain Haiss. _a._ Shales and sandstones with _Exogyra_, _Arca_, _Cucullaea_, etc., _Neolobites vibrayeanus_, etc. _b._ Hard crystalline limestone with flints. _c._ Sandstone passing into hard quartzite. _d._ Variegated sandstones with bands of sandy limestone. _e._ Hard dark-brown siliceous crystalline limestone. _f._ Calcareous grit (≡ bone-bed) passing in part into sandstone, and capped by hard crystalline limestone with shells.] Between this point and Jebel Hefhuf the fold is marked by a line of low isolated limestone-capped, cup-shaped hills, the largest and best marked of which is 16 kilometres north of Ain el Haiss. Here the maximum dip amounts to 55° on the western side of the basin-shaped hill. Jebel Hefhuf is, perhaps, the most striking part of the fold. The southern portion owes its existence entirely to this syncline, consisting as it does of a long, and remarkably narrow, ridge-shaped hill of folded rocks. Dips of 50° and 60° are common in places, and the limestones implicated in the fold are usually rendered hard and highly crystalline. The highest crystalline limestones in Jebel Hefhuf probably represent the White Chalk, but the beds are too crystalline to show any traces of fossils.