RIVER TERRACES OF THE MARIBYRNONG RIVER, VICTORIA 57 
Age of the Keilor Plains Basalt 
The Keilor Plains basaltic lava averages about 45 feet in 
thickness, and is one of the series of flows known in Victoria as 
the Newer Basalt. 
We have recognized two phases in the Newer Basalt. During 
the earlier one, the great lava flelds such as those of the Keilor 
Plains and the W estern District were formed. The later or scoria 
cone phase extended well into the Recent, and during it, scoria 
cone flows issued from points of eruption on the great lava flelds 
and the highlands to the north. The Keilor Cycle of erosion 
commenced on the Keilor Plains lava field immediately after the 
lava covered the area. There is, however, a difference of opinion 
as to the age of this lava field. Hills (1939), on physiographical 
evidence, came to the conclusion that it was certainly post- 
Kalimnan (Lower Pliocene) but that there is nothing to indicate 
whether it is Pliocene or Pleistocene. In consequence of this 
uncertainty, we have fixed the age of the Maribyrnong River 
valley cycles by working backwards from the last eustatic 15 to 
20 feet fall of sea level (indicated by platforms and ridges on the 
Yarra Delta and the shores of Port Phillip Bay), not by working 
forwards from the Keilor Plains lava field. Nevertheless, the 
evidence as to the age of the lava field, which restricts the down- 
ward extension of the physiographic cycles, seems to support our 
correlation of the cycles, and we have discussed it at some length. 
Immediately beneath the Keilor Plains basalt is a widespread 
bed (referred to here as the “Sub-Basalt Sands”) that is 
predominately sand, but sometimes consists of clays, sandstone, 
grit, pebbles, etc. It has a thickness of about 40 feet in the 
neighbourhood of Keilor, but about 80 feet south of the Ascot 
Yale Gap. It was evidently deposited on a peneplain, and after 
it was deposited, presented a peneplain surface to the enveloping 
lava. The only fossils recorded from it (Crespin, 1926) are 
unrestricted freshwater mollusca, %Cyclas or Unio, and sponge 
spicules, Spongilla. 
The nearest place to the Maribyrnong River valley where 
evidence of the age of the Newer Basalt can be ascertained with 
some certainty is at the Moorabool Viaduct, about 30 miles to the 
south-west. Here the Newer Basalt overlies ferruginous sand- 
stone, which rests on marine fossiliferous Werrikooian or Upper 
Pliocene beds. Singleton (1941) discusses the viewpoints held 
by Dennant; Tate and Dennant; Tate, Hall and Pritchard; 
Dennant and Kitson; Chapman; and Singleton, on the age of 
these fossiliferous beds ; these differ mainly as to whether it should 
B 
