541 



is sandy and of greater depth than on the plain, and, in those 

 seen by us, free from, limestone. In addition to the dongas 

 there are numerous '"bloAv holes," fissures of varying depth 

 in the limestone, that open in many cases into caves below. 

 It seems probable that the dongas represent areas of sub- 

 sidence.'-^) Tlie whole structure of the ulain is such that the 

 rain which falls readily disappears below the surface. Only in 

 the dongas is there sufficient soil to hold an appreciable water 

 reserve; the soil of the plains is too shallow. 



Startino- at Ooldea and running eastwards for about 50 

 miles is a sajidhill region, consisting of a series of ridges of 

 red-coloured sand with flats between. The ridges, which may 

 be as much as 30 ft. in height, run approximately north-west 

 by south-east at Ooldea, but there are many irregularities 

 and connecting ridges. The sand forming the ridges is rela- 

 tively stable and no drifting of large masses occurs. Indeed, 

 some, at any rate, of the larger ridges have a core of travertine 

 limestone 3 ft. or so below the surface. The soil of the inter- 

 vening flats is generally level and composed of finer particles, 

 and is consequently firmer. This soil, on the whole, appears 

 <leeper than on the ridges themselves. Whatever be the 

 source of the sand, it is clear that a certain amount of wind 

 sorting has taken place, with the result that over the whole 

 area two habitats differing in their edaphic conditions have 

 been developed. 



About three miles north of Ooldea lies the famous Ooldea 

 Soak. This is a shallow basin, 10 acres or more in extent. 

 The sand of the Soak and its surrounding ridges is whiter 

 and less stable than that of the majority of the sandhills; 

 it rests upon an underlying layer of bluish clay. Within the 

 area there is considerable drift, so that the floor of the basin 

 i:S. broken by various hollows and ridges held by shrubs. Below 

 the surface, at a depth of 5 ft. to 15 ft. or more, lies a water 

 table, the water being fresh, somewhat alkaline, but generally 

 quite potable. The Soak has been the scene of human activi- 

 ties for a great period of time, for it was known to the 

 aborigines long before the explorer, E. Giles (who was one of 

 ' the first whites to visit it), used it as a base camp in 1875.^^) 

 Later it was used as a centre for sheep grazing, (^^ but appears 

 to have been abandoned for that purpose some time before 

 the building of the Transcontinental Line made it a place of 



(5) Brown. H. Y. L., quoted Proc. Roy. Geog. Soc. S. Austr., 

 loc. cit., p. 134. 



(6) Giles, E., Austraha twice Traversed, ii., p. 152, 1889. 

 (v)Brown, T., Proc. Roy. Geog. Soc. S. Austr., loc. cit.. p. 149. 



