58 RIVER TERRACES OF THE MARIBYRNONG RIVER, VICTORIA 
be assigned to the Upper Pliocene or Lower Pleistocene. He 
summarises the evidence by remarking that “a critical study has 
shown that five per cent, of the mollusca are extinct species, so 
that the Werrikooian may be placed in the uppermost Pliocene, 
immediately preceding the Pleistocene, with a molluscan fauna 
of living species only.” As the Newer Basalt at the viaduct rests 
in a shallow valley in the ferruginous sandstone (Pliocene- 
Pleistocene) — a valley that represents a short time interval 
between the depositing of the sandstone and the extrusion of the 
basalt, Singleton’s summary leads to the conclusion that the 
basalt is Pleistocene. Nevertheless, although the Keilor Plains 
and the Moorabool Viaduct basalts have been assigned to the 
Newer Basalt, they are so far apart that we cannot be certain 
that the flows are wholly contemporaneous. 
It would be possible to fix the age of the Keilor Plains basalt 
if there wei'e means of estimating the time taken for the vertical 
erosion of the Keilor Cycle. However, assuming on the incon- 
clusive evidence that the basalt is Pleistocene, and the sequence 
of the Maribyrnong River valley flood plains and terraces is 
Upper Pleistocene, the lava field is Middle Pleistocene. 
FIG. 4. 
Levels between Dry Creek and Williamstown of Contact of Lower Surface of Keilor Plains 
Basalt with Sub-Basalt Sands. 
Warping and Tilting 
Although the Keilor Plains lava field is warped and tilted, 
eustatic adjiistment, not tectonic movement, is regarded as 
responsible for the deposit and subsequent terracing of the 
Keilor and Braybrook Flood Plains. It is also responsible for 
the deposit of the Maribyrnong Flood Plain, and, for the most 
part, its terracing; warping has contributed to this, but to an 
insignificant extent. 
The Keilor Plains lava field, and the Sub-Basalt Sands, are 
tilted in a south-south-easterly direction. The tilting occurred 
along zones of warping approximately parallel to the north-west 
