220 



have been those of epeirogenic uplifts, block-faulting, and 

 the developments of the great rift valley. 



3. Peneplanation and Geological Cycles. 



There is perhaps no part of South Australia in which 

 long-continued atmospheric waste can be better illustrated 

 than in the region now under description. There are no 

 less than five distinct geological systems represented in this 

 small area, and yet the country possesses no lines of strong 

 relief, but maintains throughout the form of a featureless 

 plain. This is not the result of a single period of peneplan- 

 ation, but several such periods. There are tremendous gaps 

 in the geological order of succession, indicating long periods 

 during which denudation exceeded aggradation. 



(a) PRE-CAMBRIAN PENEPLAIN. 



That the Pre-Cambrian complex had been reduced to a 

 comparative level before the Cambrian sediments were laid 

 down on this ancient floor is self-evident from the common 

 level which this floor generally maintains under the Cam- 

 brian deposits. The Pre-Cambrian schists are usually nearly, 

 or quite, vertical in position, as compared with a relatively 

 low angle in the case of the Cambrian beds that rest upon 

 them, indicating an incalculable interval of time between the 

 respective geological systems. This comparison applies to 

 the beds on the mainland equally with those on Yorke 

 Peninsula. The Pre-Cambrian floor has been broken, tilted, 

 and rifted down, in various segments, under tectonic move- 

 ments ; but in this floor we have the base level of the original 

 cycle of erosion in the history of this continent — the first great 

 cycle of peneplanation of which there is any record. 



(b) PALAEOZOIC PENEPLANATION. 



The loss that the Cambrian beds have undergone by 

 denudation in the region now under description is excessive. 

 That these beds originally covered the whole of what is now 

 known as Yorke Peninsula admits of little doubt, but at 

 present they only exist as scattered fragments. The most 

 southerly outlier occurs at Curramulka, a narrow coastal 

 strip is seen at Ardrossan, a few scattered stones were 

 observed on the western side of Yorke Valley, and they were 

 proved in the Maitland No. 1 Bore, where they are obscured 

 in their outcrop by a mantle of surface material. They 

 gradually make larger surface features as they pass to the 

 northward at Dowlingville, Montgomery's, Clinton, Kainton, 

 Port Hughes, Wallaroo, Kulpara, the Hummocks, etc. 



