281 



Towards the end of the clay banks, on the sea front, the lower platform 

 changes its character ; instead of clay, sand, and grass, there is a platform com- 

 posed of very large sea-worn boulders that is now about 5 feet above high tides. 



Cambrian Rocks in Section A. 



The first evidence of the older rocks outcropping on the coast occurs where 

 beds of Cambrian age show themselves in the cliffs opposite to the Marino rail- 

 way station. These beds form part of the faulted segment which, inland, form 

 the platform on which Blackwood and Belair are situated, and is next above the 

 sunken Adelaide segment. No beds of the (older) Adelaide Series reach the 

 coast within the area now under description, but the "Brighton limestones," 

 which form the topmost members of that Series, make a line of outcrop on the 

 rise, about half a mile from the coast, and can be traced southward to the River 

 Onkaparinga, where they cross the latter about a mile, up-stream, from Noar- 

 lunga. The junction of the Adelaide Series with the Cambrian beds can be 

 seen along the line of excavation in the quarries of the Brighton Cement Com- 

 pany's works. The limestones have a thrust to the westward and roll on the 

 dip, as well as pitch on the strike, in a succession of anticlines and synclines, and 

 are, finally, thrown down to the west, in an almost vertical position with the 

 purple slates of the Cambrian, in superior order, and, apparently, stratigraphic- 

 ally conformable with the limestones. 



The sea-cliffs at Marino Rocks consist of purple slates that are interbedded 

 with thin, reddish quartzites and limestones, having a strike almost parallel 

 with the coast. The great westerly thrust has caused meridional folding, often 

 reaching a high angle of dip. The parallel strike which the beds have to the 

 beach, together with the high angle of dip, makes a weak structure of resistance 

 against the action of the sea, which, in places, is making rapid encroachments. 

 This is well seen at Black Point, at the northern end of Hallett's Cove, where the 

 western limb of an anticline in the purple slates has become undermined, so that 

 large segments of the rock face slip down to the beach by the force of gravity, 

 littering the beach with great cubical masses of rock. On the southern face 

 of the point the sea has made a breach through the anticline, exposing a fine 

 transverse section of the folds, and has excavated the centre of the anticline 

 so as to form a cave. It is here seen that the anticline has been fractured along 

 the axis of the nip. 



At the same place excellent illustrations of horizontal slickensides (blatter) 

 occur, caused by rock movement. Finely-laminated layers of chloritized quartz 

 have formed along the planes of movement, the laminae showing by the varying 

 directions of the striae, the differential movements that have taken place in 

 the process of folding and thrust. 



A gritty limestone, of a very persistent character, occurs in the purple slates 

 near the base of the series. It does not appear in the sea-cliffs, as its line of 

 strike is a little inland — parallel with the coast. It can be seen exposed in the 

 gullies near Marino, also at Hallett's Cove, and is extensively developed near 

 Noarlunga, where it is quarried for road metal. It also occurs about the same 

 geological horizon on the western side of Spring Creek, Mount Remarkable, as 

 well as near the east and west fault at the south-eastern base of the last-named 

 mount. 



Permo-Carboniferous (Glacial) Rocks in Section A. 



The glacial features of Hallett's Cove need not be described here, in detail, 

 as they have been dealt with elsewhere (Tate, Howchin, and David, 1895; 

 Howchin, 1895). They cover an area more extensive than the Cove itself, 

 capping the cliffs both north and south of this break in the coastline, and are 

 also exposed in the banks of creeks adjacent to the Cove. 



