The Geology of the Darling Scarp at Ridge Hill. 
127 
IV. SUMMARY AND CONdA SJONS. 
(a) Geological lii^torg , — The geology of the area has been described and 
the geological history may be summarised as follows: — 
(i) The oldest rocks exposed are granites, of wliicli there are two 
main phases: — a coarse-grained ])oi‘pliyritic and slightly gneissic granite 
and a. medium even-grained massive gi'anite. The gneissic type is the 
older of the two but Imth are considered to be comagmatic and to belong 
to the Younger Granite period (late Archaeozoic). 
(ii) The granites have been considerably sheared after their emplace- 
ment. These shears, because of their Pre-Cambrian age, cannot be related 
to the hypothetical Darling Fault. 
(iii) In Proterozoic tim(‘s igneous activity is re 2 )resentcd by the intru- 
sion of epidiorite dykes. 
(iv) There' is a complete blank in the succession until late Mesozoic 
times at least when it is j)ro])able that the ferruginous sandstones were 
deposited on a wave-cut platform and that the eastern boundary of the 
ferruginous sandstone series represents the shore-line in these times. 
tv) The next event recorded is the formatioji of the high-level laterite 
on a peneplaned surface, probably in Miocene limes. 
(vi) An uplift of the aiea of the order of 400 feet took place in late 
Miocene times and diiferentinl erosion of the soft rocks (Mesozoic and later) 
to the west and the hard rocks (Pre-Cambrian) t(j the east led to the forma- 
tion of a low lying coastal i>lain (the liidge Dill Shelf) or alteniativel^', 
if the ferruginous sandstones are not of iJesozoic age, tlie developnaait by 
marine erosion of a marine platform (the Ridge Hill Sliclf) covered with 
a thin veneer of beach deposits wliicli have later been cemented with fer- 
ruginous material to yi(‘ld the ferruginous sandstones and conglomerates. 
(vii) The area was then elevated slightly until the Ridge Hill Shelf 
stood slightly above sea-level and the low-levcl Uiterite developed in situ on 
this newly emerged terrain. 
(viii) The area has since been raised approximately 250 feet, after 
which much of the ferruginous sandstone series was removed ))y erosion, 
especially that }')ai’t whicli previously extended across tlie present Helena 
valley. 
(ix) Contem]mraneons]y with these upward inov(>meuts of the plateau 
to the east there was continual subsidence (downwai'ping) of the area lying 
to the west of the Scarp and dc])osition in this subsiding trough of the 
Tertiary deposits of the Swan Coastal Plain. 
(x) In comi)aralively recent times tlu' yellow sands have accumulated 
either as aeolian deposits blown against tin* erosion escar|)ment capped by 
the low-level laterite or l>y deiiosition of the Coastal Limestone formation 
against this escari)ment and the subsofiuent leaching of the calcareous cement 
yielding th(* structureless, unconsolidated yellow sands. As has been in- 
dicated on a ]n*evinus |)age these sands are not residual from the disintegra- 
tion of the ferruginous sandstones. 
(xi) Laterite formation aj^pears to be taking place within the yellow 
sands at the ])resent day. 
