46 RIVER YARRA, VICTORIA 
levels indicates that the flow or flows there are of Pleistocene age. 
The lignite, shelly marl, and such beds are evidence of alternation 
of conditions. The high eustatic levels would bring about the 
deposition of estuarine beds. The Sorrento Bore also provides 
ample evidence of alternation of conditions (Chapman 1928, Keble 
1946). 
GENESIS OF Port PHILLIP Bay 
The bay owes its origin chiefly to Selwyn’s Fault, which 
developed probably in Holocene times and is still active. The 
fault runs along the eastern margin of much of the bay (Keble, 
1946, fig. 2), and has brought about a block-tilting effect which 
allowed encroachment by the sea. The crowding of the submarine 
contours on the eastern side of the bay (see Keble’s figure 2) is 
probably due to the faulting plus the scouring developed thereby. 
However, if the bay originated by faulting alone, the deepest 
water would be along the fault line, but this is not so. The deepest 
water is in the middle of the bay. This is due to the fact that 
a deep and wide valley was carved out during eustatic low sea- 
levels, so that when the sea came to its present level a large 
estuary had already been formed. The fault has increased the 
area of encroachment. Keble (1946) has given the name Bellarine 
Fault to the hinge of the tilt-block. 
In late Pleistocene times, dune building established a bar across 
the present Port Phillip Heads. The mouth of the Yarra migrated 
to different places between Mount Arthur and the Bellarine Pen- 
insula, for as one exit was blocked by dune-building, another had 
to be found. Keble (1946) has described the Bay Bar and the 
various debouchements which can be traced in the submarine 
contours. The infilling of the Pleistocene valley of the Yarra is 
still proceeding in the bay, although negatived to a certain extent 
by movement along Selwyn’s Fault. 
The formation of Port Phillip Bay by flooding of the Pleis- 
tocene valley and movement on Selwyn’s Fault betrunked the 
Yarra river system, so that streams which once flowed into the 
Yarra now debouch into the bay. 
NEWER BASALT CYCLE 
The present cycle of erosion was precipitated by the extrusion 
of the Newer Basalts. The Upper Yarra and Middle Yarra were 
not affected, and thus their courses are much older. The Lower 
Yarra was forced against its southern valley wall, and its thal- 
weg raised considerably. The gravels and other fluviatile deposits | 
