40 RIVER YARRA, VICTORIA 
The figures in the above table show an average declivity in the 
thalweg over the two miles between Punt Road and Spencer Street 
of 12-6 feet per mile. If this average declivity is assumed for the 
36 miles from Spencer Street to the Port Phillip Heads, then the 
pre-Older Basalt river there must have been 454 feet lower, i.e., 
536 feet below datum. The declivity of a stream is commonly 
reduced in its lower reaches, especially as the coast is reached. 
Government 
HOVSE HILL 
Half Mile. 
FIG. 7 
Map to indicate positions of bore traverses across the River Yarra at Melbourne. 
1 is Punt Road, 2 is Swan Street, 3 is Russell Street, and 4 is Spencer Street. 
O.B. = Older Basalt. N.B. = Newer Basalt. 
However, the coast then was probably in the vicinity of Cape 
Otway (as in the Pleistocene), and 12-6 feet per mile is already a 
low declivity, i.e., 1 in 420. So the figure arrived at is probably of 
the right order. 
In a previous paper (Gill, 1942), the declivity of the Wurun- 
jerri River was calculated to be 15 feet per mile. Applying this 
figure to the 56 miles from Lilydale to Port Phillip Heads gives 
840 feet, from which must be subtracted 378 feet which is the 
elevation of the thalweg at Lilydale above sea-level, viz., 462 feet 
below datum. 
