57 



took place in the interval between the deposition of the- 

 Eocene and that of the Miocene, and if so the Miocene laid 

 down at Sellick's Hill must have shown a much stronger 

 unconformity with the older Cainozoics as compared with the- 

 Port Willunga section. 



The next newer system represented in the Sellick's Hill 

 section is a thick accumulation of Pliocene (or Pleistocene) 

 clays and gravels, which rest uneonformably on the Eocene 

 beds. These alluvial cliffs rise precipitously to a height of 

 200 ft., and are deeply scored by rain and surface drainage. 

 They are, geologically, undisturbed, and, in places, occupy 

 lines of erosion in the Eocene limestones. We can thus 

 narrow down the limits of the period of tectonic activity as 

 post- Eocene and pre-Pleistocene. This brings it somewhere 

 within the Neogene period, but whether as an inauguration 

 of the Miocene, or as characteristic of some inter-Miocene 

 period, or, as marking its close, or even as Pliocene, we have 

 not at present the data to decide. 



The process of disruption probably began in the form 

 of a dome-shaped regional uplift that included most of the 

 southern portion of South Australia. In this upward move- 

 ment a degree of strain was reached when the rising dome 

 became intersected with fractures, and was split up into 

 vast blocks of country, which, being unequally supported 

 settled along some lines and left others strongly in relief. 

 This process of block-faulting would result in major and 

 minor effects. The great slopes of Mount Lofty to McLaren 

 Vale, and the Willunga scarp and plateau, represent some 

 of the major lines of disruption, and these, again, are split 

 up into secondary blocks, scarps, and trenches which make 

 the minor features of our landscapes. It is very unlikely 

 that these diastrophic effects were produced by a sudden 

 or cataclysmic occurrence, but resulted, no doubt, from a 

 number of small movements, spread over a long period of 

 time, and may even still be in progress. 



The downthrow to the gulf, seen in the Sellick's Hill 

 section, supports the view of the existence of a great trough- 

 fault, or graben, in the line of Gulf St. Vincent — a view 

 which has already been assumed by the writer as necessary 

 for the explanation of other local geological phenomena. It 

 is very probable that the earth tremors which occur in the 

 southern portions of the State are connected with these great 

 lines of crust fracture. In the important earthquake of 

 September 19, 1902, the foci of maximum intensity was in 

 Gulf St. Vincent, opposite to the disturbed area described in 

 this paper, and the tremors were particularly severe in a 

 line facing the coast, and also in the valley along the base 



