36 PLANT HABITS AND HABITATS IN THE 



ture of the western side of Lake Eyre. "The regular flat-topped hills 

 represent the old land-levels, fragments of which have been preserved 

 from denudation by hard silicious beds that have formed the protecting 

 cappings for the softer beds beneath" (Howchin and Gregory, I. c.). 

 At present this formation is reduced to a triangle, of which the base 

 abuts against the northern end of the highlands of South Australia 

 (Gregory, 1906:64). On the north it extends apparently beyond the 

 confines of South Australia and on the west it reaches to the eastern 

 end of the Everard Ranges. The desert sandstone belongs to the 

 Upper Cretaceous series. According to Jack (1915:41): 



"The series is made up of sandstone, grit, light to grey shale, and a little 

 limestone and, as far as it is possible to judge, has a thickness not exceeding 

 200 feet. * * * Subsequent to the elevation of these beds striking hydro- 

 chemical metamorphosism has taken place under the joint influence of light, 

 rainfall, and warmth, resulting in very extensive silicification of the surface 

 rocks * * * largely responsible for the very characteristic topography 

 of the Upper Cretaceous areas. * * * The presence of this resistant 

 capping of quartzite, chert, or flint has resulted in the formation of table- 

 topped hills and tablelands. The wearing away of the soft underlying shales 

 undermines the indurated beds, which break up to form the stony mantle of 

 the ' gibber ' plains, and by its presence affords evidence of the former extension 

 of the Upper Cretaceous rocks, even though denudation has proceeded so far 

 as to leave the residual stones resting directly upon the blue shale of the Lower 

 Cretaceous series." 



The author goes on to say that the rounded form of the "gibbers" 

 is due to the action of insolation. Many of the stones are highly 

 polished as a result in part from the attrition of dust and sand blown 

 by the wind, and in part from a coating on their surfaces of a glaze 

 caused by the evaporation of siliceous and ferruginous water on the 

 stones. This desert glaze, or varnish, Jack states is a feature common 

 to arid regions. Largely because of the reflection of light from their 

 polished surfaces, but also because of their presence under foot, the 

 gibber plains present difficulties for the traveler. For example, Spen- 

 cer and Gillen (1912:40) narrate: 



" A little way to the north of Oodnadatta we passed on to gently undulating 

 country, with low-lying, flat-topped hills and remarkable plains covered with 

 small stones. Nothing could possibly be more desolate than these 'gibber 

 fields.' * * * The horizon is shimmering and indistinct and the level 

 ground is covered with a layer of close-set, purple-brown stones, all made 

 smooth and shiny by the constant wearing action of wind-borne sand grains, 

 for, in winter especially, a strong southeast wind often blows all day long." 



As these excerpts would indicate, the desert sandstone plateau con- 

 stitutes, for various reasons, a feature of the landscape of the more 

 arid portion of South Australia that is striking in the extreme. 



The mountains and the hill country of South Australia are of two 

 classes, geologically unlike and to a degree constituting separate physio- 



