The Geology of the Darling Scarp at Ridge Hill. 
115 
The Lower Cretaceous (?) sandstones of Bullsbrook are lithologically 
similar to the Ridge Hill ferruginous sandstoneSj as they contain both 
angular and well-rounded sand grains. A detailed examination of the 
roundness and surface textures of the grains of the Bullsbrook sandstone 
has not yet been made. 
Text figure 7 illustrates diagrammatically the structure of the Swan 
Coastal Plain on the assumption that the Ridge Hill ferruginous sandstone 
is of Lower Cretaceous age. It may be noted here that Maitland (1919, p. 
6) records that the Helena River carries 22,000,000,000 gallons less past 
Midland Junction per annum than it does further ujtstream near (Ireen- 
niount where it is still on the Pre-Cambrian complex. It is probable there- 
fore that this water enters Ihe Coastal Plain Artesian Basin through the 
ferruginous sandstone series. The unconformity between these ^lesozoic 
and the Kainozoic rocks of the Coastal Plani may be of the nature of an 
overlap. These ferruginous sandstones, extending along the front of the 
Darling Scarp, inay therefore be the main channel from which rvater is dis- 
tributed to the various a(|uifers in the rocks, ranging in age from Lower 
Cretaceous to late Kainozoic (Parr, 1938, p. 71), which underlie the iMetro- 
politan Area but. so far as Is known, do not outcrop. 
The ]>eri(}d of formation of the i)resent Ridge Hill shelf is later than 
that of the high-hwel laterite which was probably Miocene according to 
AVoolnough (1918). It rvas formed when the laterite-covered plateau area 
to the east had been elevated to aj>proximatcly 409 feet above sea level, i.e 
present elevation of high-level lalerite (700 feet) minus the elevation of 
the Ridge Hill Shelf (300 feet). If the ferruginous sandstone series be of 
Lower Cretaceous age then it represents an exhumed Lower Cretaceous 
shoreline with a wave-cut bench covered with marine sands; if of post- 
Miocene ag'e then it is a marine wave-eut b(mch with a thin veneer of beach 
deposits which have subsequently been cemented by iron-bearing solutions 
into a ferruginous sandstone, 
(2) The Ln/cr/tc.s.— Laterite occuj's at two distinct levels— the high-level 
laterite iu the south-eastern corner of the area at an elevation of 700 feet 
^lbove sea-level and the low-level laterite on the Ridge Hill Slielf at eleva- 
tions of 220 feet— 280 feet above sca-lcvel. As has been pointed out in the 
introduction to this paper AVooluough regards these two laterites as being 
of the same age and origin, their differences in elevation being due to block- 
faulting. Simpson (1912, p. 400) considers that, broadly speaking, there 
are two cla.sses of laterite in M eslern Australia, fir.stly the primary (or 
high-level) laterites and secondly the secondary or low-level laterites occur* 
ring at lower levels and composed largely of mechanically transported frag- 
ments derived from the high-level laterite. I am not aware whether or no 
Simpson had in mind the lo^v-lcvel laterites fronting the Darling Scarp in 
his mention of secondary laterite but it seems from bis description that he 
would regard the low-level laterite of Ridge Hill as a secondary laterite 
(lateritite). 
Field map})ing has shown that in this area the high-level laterite 
Is developed over the Pre-Cambrian complex whereas the low-level laterite 
has developed over the ferruginous sandstones described in the previous 
■section of this paper. ]Moreover the low-level laterite appears to be a 
