president's address. —section c. 171 



of an old river, surrounded by open country, and is a sluggish stream 

 which for long distances has been reduced, locally, to base-level. In 

 the lower half of its course it exhibits totally different features — 

 flowing through a narrow, rocky gorge, 300 feet deep, having a steeper 

 grade, and with the typical features of a relatively young river. Here 

 again, as in the case of the Barossa country, we have the effects of 

 block tilting on a large scale. The Onkaparinga is an antecedent 

 river, being a survival from the older systems of drainage. The anomaly 

 of its features can only be explained on the grounds of differential 

 earth movements. While the head of the river is little altered from 

 what it was in the older system, the lower portions have been rejuve- 

 nated as the result of a tilt in the uplift. The earth movement, how- 

 ever, was not of the nature of a simple uplift, for the Onkaparinga 

 has had a very chequered history. In the first stage of its existence 

 it was a noble river that took a direct course over the Kangarilla 

 Flats and McLaren Vale, and along the base of the Willunga scarp, 

 finally uniting \vith the trunk valley (now drowned) near Sellick's 

 Hill. At the point where it made its junction with the main stream 

 magnificent faces of river gravel, 300 feet high, form the sea clifis. 



Before the last great uplift, that either created new consequents 

 or rejuvenated old streams, there was a period of depression under 

 which the older Onkaparinga lost grade, and gradually aggraded its 

 bed with an accumulation of load that it was unable to transport. 

 The downward movement continued until it not only completely 

 choked its own valley, but overspread the surrounding country, which 

 was then reduced to base-level, and the river meandered over extensive 

 flood plains. When the great uplift began, the river, which throughout 

 its lower portions had forsaken its original course, began to cut its 

 way down through its own deposits, and then touched rock into which 

 it has cut one of the deepest and most rugged gorges in South Aus- 

 tralia. The Onkaparinga, therefore, in its lower sections, is an incised 

 meander, its course being determined by the direction which the stream 

 had when at base-level and at the time when the reversed movement 

 of uplift began. As a consequence of this the Onkaparinga finds its 

 way to the sea some 12 miles further north than was the case in its 

 earlier stages. 



The interval of time separating the present from that latest im- 

 portant earth movement that caused the elevation of the Mount Lofty 

 Ranges must be represented by the equivalent of work done by river 

 erosion, as just illustrated, in cutting a gorge of 300 feet or more in 

 hard Cambrian slates and quartzites. 



HYDROGRAPHIC AND CLIMATIC CHANGES. 



The earth-movements that occurred in South Australia during 

 the later Cainozoic periods produced certain hydrographi and climatic 

 changes which profoundly altered the face of things. South Australia 



