130 . 
S.A. NAT., VOL. XIV. 
The Coast from Outer Harbour to Sellicks Hill. August 31st, 1933. 
The reason for the rhythmic arrangement may be seen in 
the block diagram. Four different kinds; of rock are found in 
t>bis area. — (a) The hard, ancient rock, shown by nearly vertical 
wavy lines, Cambrian and Pre-Cambrian in age, which forms the 
ranges, the level uplands, and the more notable of the marine 
■cliffs. In ages long past these ancient rocks were worn down to 
a level surface by the slow-acting forces of wind and rain and 
running water. They were then sunken gently beneath the sea, 
and some hundreds of feet of limestones were formed by the 
remains of the sea animals deposited upon them. 
(b) A gradual rising movement followed and these lime- 
stones, laid down on the sea floor, became dry land. Naturally 
they are rich in fossils. They are shown in the block diagram 
as horizontal beds without marking and are called Miocene lime- 
stones. They form the yellow cliffs of Noarlunga, Aldinga, etc, 
(c) Following on this a series of mighty disturbances in the 
earth’s crust took place hereabout, so that huge fractures hun- 
dreds of miles long and possibly some miles in depth run through 
these ancient rocks and their covering limestones. Some of the 
intermediate blocks of rock were sunken and others were up- 
raised. Erosion took place, aud vast deposits of red muds were 
here and there laid dov'ii covering the limestones. Sucii red 
muds are still to be seen in the cliffs at Seacliff. llalletts Cove, 
Noarlunga, and Sellicks Hill. They were formed by fresh water 
and are mostly red or brown, but sometimes green or yellow, in 
colour. They are marked in the map by vertical lines and are 
called Pleistocene muds and sands. 
(d) The great faulting and uplifting movements continued 
and the uplifted or mountaifi blocks in this area became tilted 
downwards to the south, as you may see in the two central blocks 
of the diagram. A good deal of the limestones and mudstones 
was washed off and carried away, but close to the fracture or fault 
lines still newer beds of alluvial material were deposited to form 
wide plains. I'hese are shown in the map as recent alluvium. 
We may now see the reason for the rhythmic arrangeinenL 
above referred to. Where soft muds and sands form the sea 
beach the sand is washed clean and blown up to form a barrier 
of sand-dunes. Where the limestones reach the sea, soft cliffs 
of yellowish or brownish limestones border the coast. Where 
the ancient hard rocks meet the sea, the latter carves out steep 
cliffs and shingly beaches with much difficulty, for these old rocks 
uc intensely hard and resistant. 
