52 ' PLANT HABITS AND HABITATS IN THE 



also mention the sandhills and accompanying clay-pans situated 2 or 3 

 miles east and south of Oodnadatta. The sandhills he on the lower 

 plain and vary considerably in plan, extent, and height. The highest 

 seen were estimated to have an altitude of about 15 meters above the 

 plain and to be about 50 meters in diameter. They are round, oblong, 

 or crescentic in plan. The windward slopes of some are gradual and 

 the lee slopes steep. Ripple marks also are to be seen on most of the 

 dunes. From such features it is concluded that not all of the sandhills 

 are fixed. Adjoiniag the sandhills, or between them, are clay-pans 

 which vary greatly in size, some being 1,200 meters, more or less, in 

 diameter. The surface is hard and glistens in the sunlight. Although 

 when seen the clay-pans were entirely dry, whenever rains occur they 

 are covered with water which apparently escapes, mainly by evapora- 

 tion, inasmuch as the surface of the pans is of fine texture, apparently 

 silt, which would allow but relatively slow penetration. 



In one group of sandhills the clay-pans appeared to extend beneath 

 the dune, although this was not surely determined. If true, it would 

 follow that under such a condition there might be important water 

 reservoirs in the sandhills and at no great depth, "Soaks" among 

 the sandhills, as possibly at Ooldea, may have such an origin and struc- 

 ture. Howchin (1909:103) says that sandy ridges and clay-pans form 

 the southern plains of the central portion of Australia and occur 

 largely on the eastern and northern sides of Lake Eyre, and points out 

 that such an ialand basin as that of Lake Eyre can not get rid of its 

 worn-down material, the product of erosion, "such as occurs when the 

 drainage of the country flows into the sea, whilst from a deficiency of 

 moisture, vegetation is scarce and the soil but loosely held together. 

 From this cause the soil and sand are constantly on the move." 



From the foregoing sketch of some of the leading physiological 

 features in the vicinity of Oodnadatta it will be clear that the character 

 of the soil varies considerably. On the plains, for example, it appears 

 to be fine sand with an admixture of pebbles of different sizes. The 

 largest of these constitute the "gibbers," or desert pavement, which 

 protect the fine soil beneath from being carried away by the wind and 

 from rapid drying out. In the shallow hollows on the plains, as has 

 already been alluded to, there are no "gibbers." This also is true on 

 the flood-plain of Neales River, where, however, the small stones are 

 largely wanting and the soil is relatively sandy. On the plains the top- 

 soil appears to be about 50 cm. deep, and on the flood-plain it is 2 

 meters, and probably much more, in depth. 



In places on the plains where erosion has occurred and the substratum 

 is exposed, it is seen to be of a white color and often of considerable 

 thickness. How generally this extended was not learned. From the 

 appearance of the material it was assumed to be gypsum, although in 

 certain places where the layer of material was thin, as where it covered 



