ARID PORTIONS OF SOUTH AUSTRALIA. 51 



the western edge of the great Australian artesian basin. It is about 100 

 miles to the north of west of the northern end of Lake Eyre. The 

 most important topographical feature of the immediate neighborhood 

 of the town is the lower plain (Lower Cretaceous) on which it is situated. 

 Outlying members of the higher plain, the desert sandstone tableland, 

 can be seen to the westward (plate IA). These flat-topped hills are a 

 striking feature of the topography of the region. The surface both 

 of the lower and of the upper plain is usually covered with fairly small 

 stones, the gibbers, which were left by the wind after all fine material 

 had been removed (plate IB). Although a superficial view over the 

 lower plain gives the impression that it is quite flat, when seen more 

 closely such is found not to be the case. It is slightly undulating and 

 there are shallow depressions, apparently wind-scooped, here and 

 there, where gibbers appear to be wanting, and which are but a few 

 inches below the general level of the plain. Such, as will be mentioned 

 below, are in fact miniature oases which support a characteristic 

 vegetation in the midst of a plain otherwise quite barren. 



So far as I saw, the surface of the flat-topped hills, which are the re- 

 mains of the upper plains, it is much like that of the lower plain, 

 except that the effects of erosion are more marked. There are small 

 depressions in this plain also, and it is covered with gibbers. Possibly, 

 as will be remarked later, the upper plains constitute the most arid 

 habitats of the great basin. 



The most prominent water-course in the vicinity of Oodnadatta' is 

 Neales River. At Oodnadatta the river-bottoms are defined by low 

 banks, and are possibly 6 feet below the general level of the lower plain. 

 But as the river goes southeast on its way to Lake Eyre, the course 

 widens and is not so sharply delimited as it is near Oodnadatta. 

 Neales River and the Macumba River constitute the only rivers leading 

 into Lake Eyre from the western side of the basin. At a point west of 

 Oodnadatta and about a mile distant, the bottoms of Neales River are 

 nearly a mile across. The surface of the bottoms, or flood-plain, is 

 level. There is no continuous and well-defined water-course here, 

 but at various places along the bottoms are depressions, 100 meters or 

 less in length, which contain water after rains. These depressions have 

 the appearance of having been gouged out by water at the time of high 

 floods, which rarely occur. It is doubtful whether the water of the 

 rivers ever empty into Lake Eyre. Just north of Oodnadatta there 

 are flats which may possibly drain into the Neales River ; and several 

 miles farther north, as viewed from the summit of O'Halloran's 

 Mount, there are extensive flats also, with narrow contributory chan- 

 nels, which probably act as reservoirs for the reception and catchment 

 of flood-waters. All of these are probably related to the drainage of 

 the main river. 



Besides the plains and the drainage channels of whatever kind, in 

 connection with the topographical features of the region one should 



