president's address. — SECTION c. 161 



the crust of the earth has sunken, meridionally, in long sections, and 

 gone down in successive steps towards a deep trough, or rift, that 

 constitutes one of the most striking features in the geological structure 

 of South Australia. In the neighbourhood of Adelaide four such 

 faulted platforms can be recognised (see Plate IV.). 



First Fault-Platform. — Five hundred feet lower than the general 

 level of the Mount Lofty plateau, and about 1,000 feet above sea-level, 

 there is an important shelf of Cambrian rocks, corresponding, in a general 

 way, to the average height of the piedmonts. This shelf forms the 

 highest of the series of western step faults, and at this elevation an old 

 base-level was developed when the main drainage was north and south. 

 Thick alluvial deposits of mottled sands and clays and occasional gravel 

 beds (corresponding to the upper miocene of Tate) occupy the crests 

 of the foot-hills of the ranges. They make the surface features of 

 Belair, Blackwood, the higher levels of Coromandel Valley, and, in a 

 slowly decreasing gradient, pass through Happy Valley, Morphett 

 Vale, and Noarlunga, where, as old channels of drainage, they formerly 

 united vvdth the trunk stream that flowed down the valley now occupied 

 by the waters of the Gulf. The fluvial deposits of this dead river overlie 

 the fossiliferous marine beds during part of their course, and on the 

 sides of the valley make scarps of indurated material, 10 to 12 feet in 

 height. These old river terraces occur at intervals as far as their outlet 

 at Noarlunga. This shelf of Cambrian rocks, which carries deposits 

 from early caiuozoic times to the present, may be appropriately called 

 the Blackwood-Belair platform. 



Second Fault-Platform. — This occupies the sloping ground at the 

 base of the foot-hills. The fault-plane connected with the downthrow 

 can be seen in the Fourth Creek (Morialta) and in the quarries of 

 Stonyfell and Mitcham. The old river deposits skirt the base of the 

 hills, and can be seen at Athelstone, Magill, Stonyfell, and Burnside. 

 This may be called the Athelstone-Burnside platform, which is about 

 500 feet above sea -level and about 500 feet below the Blackwood-Belair 

 platform. 



Third Fault- Platform. — The third step-fault forms a platform of 

 Cambrian rocks at some depth below the present surface, bordering 

 the ranges and underlying the Adelaide Plains. Its position was proved 

 by the Kent Town (Adelaide) bore at 221 feet* below sea-level, or 



* Tate, " Notes on the Tertiary Strata beaeath Adelaide." Trans. R/iy. Sac. S. Aus., Vol. 

 V, pp. 40-43. Howchin, " An Outlier of Older Caiaozoic Rocks in the River Light, near Mallala." 

 Ibid., Vol. XXXVI, pp. 14-20. When Tate described the Kent Town section he had not, at that 

 time, distinguished between the Older and Newer Marine Tertiaries, and he therefore groups all 

 the marin"? beds under the Miocene. In the second of the above references I have attempted to 

 draw the line of distinction between tfe two series, guided by Tate's descriptions, but, as I have 

 not explained this in the paper referred to, it may lead to soma confusion in interpreting the table 

 given on page 20. In Tate's section, the first 53 feet of the marine beds appear to belong to the 

 upper or Miocene series ; then follow 5 feet of coarse and " very sharp " white quartz sand and 

 gravel, which appears to be the base of the Miocene, and represent the fresh- water conditions of 

 the interval of erosion ; and then, a lower marine scries, which Tate subsequently called Eocene, 

 which has a tluckness of 91 feet in the section. 



6117. V 



