24 HORN KXPEDITIOX — PHYSICAL GEOGRAPHY. 



and for many miles in an easterly and westerly direction, may be said to be 

 occupied by rocks of Cretaceous age, covered over large areas by Post-Tertiary 

 deposits and over very limited areas by Pliocene beds of lacustrine origin. The 

 upper strata of tlie Cretaceous system have been removed by denudation over 

 extensive areas, the remaining portions being in the form of more or less isolated 

 table-topped hills dotted about throughout the whole area. The materials set 

 free by this denudation have gone to form the superficial accumulations known 

 as "gibber" or "stony plains," loamy plains, and to a large extent also the 

 sandhills. 



" Stony plains " is the name given to those portions of the Cretaceous area 

 over whose surface are strewn "gibbers," i.e., rounded or subangular fragments of 

 silicified sandstone or grit known as "desert sandstone." The desert sandstone is 

 extremely hard and weather-resisting, and has a somewhat sub-conchoidal fracture. 

 With such properties it is not surprising to find fragments of it covering much 

 of the country from which the sandstone has been denuded. 



The uniform distribution of the gibbers, which is a very characteristic feature 

 of this class of country, is due to the fact that the fragments derived from the 

 layer of "desert sandstone" which extended over this area, now strewn with 

 gibbers, have not undergone redistribution to any large degree. In a flat district 

 like this the surface water has not been able to sweep them into water-courses. 

 The removal of the argillaceous sand by the wind and rain has permitted the 

 gibbers to settle down, until in many places they present a flat surface resembling 

 an artificial pavement. The outer surface of the gibbers is of a dull red colour, 

 due to the presence of a thin film of oxide of iron coating them. They also present 

 a glazed sub-angular or pseudo-waterworn appearance, produced by the polishing 

 action of the sand grains as they are driven along the surface of the ground by 

 the wind. 



The stony plains gradually merge into loamy plains, which possibly occupy the 

 sites of areas of the Upper Cretaceous, which were devoid of the " desert sand- 

 stone " capping. 



On either sides of the banks of many of tlie rivers, especially along their 

 serpentine courses over the plains, are extensive alluvial plains. During floods 

 the channels are too flat and shallow to carry oft" the immense body of water that 

 occasionally comes down them, and the water spreads out on either side, sometimes 

 for miles. The "box flats," which are met with on the sides of the Finke Channel, 

 are flood plains, on which Eucalyptus microtheca flourishes in large numbers. 



