257 



Before we answer that question there are some other facts 

 to be taken into account. The Orroroo district is situated 

 near the summit of the east and west water-parting of South 

 Australia — a broad and extensive watershed whicli divides 

 the inland drainage flowing to the north from the coastal 

 drainage which flows to the south. But this country does 

 not show the characteristic aspects of a watershed. There 

 is no rocky ridge or headwaters of a great hydrographic 

 basin to mark the water-parting. Indeed, the present water- 

 parting runs athwart the main physiographical ridges of the 

 country, and its general aspects are those of a country which 

 has been reduced to base level rather than that of a water- 

 shed. 



The only satisfactory solution of the anomaly, and one 

 that accords with a much wider circle of evidence, is that 

 of crust movements on a large scale, which at no very re- 

 mote geological period produced an east and west ridge, or 

 bulging of the surface, that dissected the drainage and di- 

 verted much of the water that originally came south in a 

 reverse course towards the northern basins. This great cor- 

 rugation of the earth's crust with its attendant warpings has 

 destroyed the trunk rivers. The smaller tributaries find no 

 confluence with other streams, and flow towards the dry val- 

 ley only to be lost in its porous and deep alluvial deposits. 

 The Walloway Creek, the Pekina Creek, the Orroroo Creek,, 

 and many others carry a considerable amount of water, but 

 are lost within a short distance after entering the plain. In 

 a well sunk at the mouth of the Pekina Creek a fragment 

 of a kangaroo femur was found in the alluvial at a depth 

 of 60 ft. 



The Siccus River takes its rise to the north of Orroroo' 

 and flows into Lake Frome. This watercourse probably re- 

 presents the reversed waters of the dead river of the Orroroo 

 plains. In the process of diversion there must have been a 

 period of arrested drainage, when the stream became sluggish 

 and unable to carry its load. Degradation gave place to 

 aggradation, and the watercourses became choked by a great 

 thickness of sediment, as is shown by the Orroroo bore and 

 the tribiitaries of Pekina Creek. 



The old lake deposits of Pekina Creek supply an in- 

 teresting ^hase in this chain of events. At the time when 

 the now extinct trunk river had reached its maximum of 

 aggradation, its bed was at a much higher level than the 

 present plain. Its alluvial spreads along the mai'gins of the 

 plain on either side of the Pekina Creek and fills in the 

 valleys of its tributary streams to a height of 100 ft. above 

 the present level of the plain. The railway, soon after leav- 

 I 



