RIVER YARRA, VICTORIA 43 
Post-OLDER BASALT RIVER SYSTEM 
The extrusion of the Older Basalt lava flows apparently brought 
about the following changes : i З à 
(1) In the Woori Yallock Basin the Wurunjerri River was 
sueceeded chiefly by a stream which flowed along the northern 
boundary of the lava field. This stream was diverted over a saddle 
in the Wurunjerri Range, and linked with a stream flowing along 
the southern margin of the Kangaroo Ground lava field. As no 
Newer Basalt lavas were extruded in these areas, this stream is 
the same as the present Yarra River. It is thus seen that the 
Yarra consists of parts of three early Tertiary rivers (or more 
accurately, their post-Older Basalt successors), viz., the Wurun- 
jerri, the Kangaroo, and the Melbourne. 
(2) The pre-Newer Basalt Plenty River has been traced by 
Jutson (1910), i.e., the stream which was established after the 
extrusion of the Older Basalt. It was probably on the eastern 
margin of the Melbourne River lava field. New streams usually 
start along the margins of lava flows, and so the position of the 
post-Older Basalt streams may, on the whole, indicate the extent 
of the Older Basalt lava field. 
(3) The Melbourne River lava field was a broad one, and a new 
stream developed down the middle of it—the pre-Newer Basalt 
Yarra. Possibly the great thickness of basalt in this field caused 
a slight slumping which would cause the water to take this 
course. 
The River Yarra flowed along the north of the Woori Yal- 
lock Basin lava field, over the saddle in the Wurunjerri Range, 
along the southern margins of the lava fields represented by the 
Kangaroo Ground and Ivanhoe residuals, through the present 
suburbs of Fairfield, Collingwood and Burnley, and so to the eity 
area, where it flowed along the edge of the Older Basalt past the 
Botanical Gardens and across the Albert Park lakes (i.e., skirting 
the Older Basalt on which South Melbourne is built), and so down 
the middle of the Melbourne River lava field to the sea. This 
course, in its lower reaches, was deeply entrenched by low eustatic 
sea-levels in the Pleistocene. 
(4) The Dandenong Creek no doubt developed as a stream 
marginal to the Older Basalt in the valley of the Wurunjerri 
River. Pleistocene low sea-levels would make this stream a very 
active one, and its work was not interfered with by Newer Basalt 
flows as in other parts of the drainage system of Port Phillip. 
Dandenong Creek has carved out a more or less circular physio- 
graphic basin north of Dandenong, because it is constricted at the 
