RIVER YARRA, VICTORIA 39 
parts of the valley. It occurs between 70 feet and 80 feet below 
datum (i.e., L.W.M. for Hobson’s Bay) at Spencer Street bridge, 
Melbourne. 
Keble (1946) has suggested that the Melbourne River flowed 
over the Bellarine Peninsula and so down towards Cape Otway. 
Further information now obtained indicates that it probably 
flowed to the east of the Bellarine Peninsula because— 
(a) Judging by the Older Basalt residuals further north, the 
thalweg of the Melbourne River was hundreds of feet below sea- 
level at Port Phillip Heads, whereas the Older Basalt on the 
Bellarine Peninsula (Daintree, 1861; Diamond Drills in Victoria, 
1885) is far too high to fit in with this physiographic pattern. 
In connection with bridge-building projects, traverses of bores 
have been made across the River Yarra at Melbourne as follows: 
Depth of bedrock in feet 
Location of bores Authority ds tin 
i Punt Road Country Roads Board SON 
il Swan Street Ditto 62:6 
iii Russell Street Ditto 70:33 
iv Spencer Street Victorian Railways 82:8 
The locations are shown in Fig. 7. The present Yarra, the pre- 
Newer Basalt Yarra, and the Kangaroo River (1.е., from early 
Tertiary times till now) have all passed over the same course in 
the area where the bores were sunk. This was due to constriction 
between the hard Silurian outerops represented at present by 
Government House Hill and Russell Street Hill (Fig. 7). 
i. The Punt Road section shows mostly silt above the bedrock, 
but a little sand is intercalated. 
ii. The Swan Street section is also mostly silt, but on the south 
bank of the river the bores penetrated basalt. 
iii. The Russell Street section reveals basalt on the north bank 
of the river. A seam of “drift sand” at about the level of the 
top of the basalt separates upper and lower silts over the 
thalweg of the river bed in the bedrock. 
iv. The Spencer Street sections have been published by Chapman 
(1929). They show in order from below up bedrock, Older 
Basalt, clay, Newer Basalt, lignite, shell marl, drift sand, and 
mud. The position of the Older Basalt indicates that the 
bedrock is the level of the pre-Older Basalt River (Kangaroo 
River) at this point, and so probably also in the other sections 
quoted. 
