RIVER YARRA, VICTORIA 41 
Comparing the figures from the Wurunjerri River (462 feet) 
and the Kangaroo River (536 feet), we may assume that at the 
site of the present Port Phillip Heads, the pre-Older Basalt river 
system was of the order of 500 feet below datum. As further 
information is obtained about the thalwegs of the above rivers and 
the other rivers of the system, it should be possible to determine 
fairly accurately the depth below present sea-level of the Mel- 
bourne River at that point. When this is done, it will be possible 
to determine how much depression is there due to eustatic low 
sea-levels and how much due to faulting along Selwyn’s Fault 
and others, if any. As the declivities are measured in tectonically 
stable areas, the difference between the calculated level of the bed- 
rock and its actual depth will be a measure of the faulting that has 
occurred. The Sorrento Bore (Chapman, 1928) penetrated 1,680 
feet of sediments and aeolian materials without reaching bedrock. 
On present knowledge, it may be said that 500 feet of this depth 
is due to eustatic emergence resulting in down-cutting of the river 
bed to that depth below present sea-level, while 1,180+ feet is due 
to faulting. 
A further check on the depth to which the pre-Older Basalt 
river system eroded below present sea-level may be obtained by 
a study of the palaeogeography of the Western Port Older 
Basalts. These are also found far below sea-level, although once 
again there has been faulting. However, the declivities can be 
worked out from the stable areas. The Western Port system 
apparently drained into the Melbourne River south-west of Cape 
Schanck, and so the depth of the thalweg of its main stream should 
fit in with those of the Port Phillip system. 
The depth of the pre-Older Basalt river system below present 
sea-level has not always been taken into account in the geological 
interpretation of some areas. For example, Older Basalt is found 
below sea-level between Mornington and Frankston, and this has 
been attributed to down-faulting. That some faulting has taken 
place is indicated by the dip of Tertiary rocks at Frankston. 
However, the depth of the Older Basalt is no doubt due chiefly to 
the position of the pre-Older Basalt river bed. 
From the foregoing paragraphs it is clear that the Melbourne 
River could not have passed over the Bellarine Peninsula, unless 
that has been up-faulted to the order of 500 feet. There is no 
reason to hypothecate this. 
(b) A second reason for considering that the Melbourne River 
flowed east of the position of the Bellarine Peninsula is that such 
a course follows the middle of the early Tertiary river valley. I 
