32 RIVER YARRA, VICTORIA 
2. That the plain is one of subaerial denudation. Alternatively, 
the Nillumbik Peneplain could have been formed by reduction to 
base-level by subaerial agencies. The slopes on the seaward sides 
and their Miocene str ata are consistent with this interpretation 
too. 
But the cause of the planation of the Nillumbik Peneplain 
needs to be considered along with the cause of the planation of 
the Yarra Plateau, and the difference i in height between the two. 
If the Nillumbik Peneplain were cut by the sea and so covered 
by it, then the Yarra Plateau could have been formed by subaerial 
denudation. However, the difference in height of 200 feet does not 
favour this interpretation. Their relationships would be some- 
thing like those of the present Port Campbell plain to the sea. 
Such difference in elevation would lead to dissection and not 
favour planation. Alternatively, the Nillumbik Peneplain and 
Yarra Plateau could have been one continuous peneplain which 
was disrupted by faulting. However, direct evidence of such 
faulting has not yet been found in the field. 
Another possible explanation of the seaward slope to the south 
of the Nillumbik Peneplain is that it was originally part of the 
peneplain, but was involved in the warping (see Singleton, 1944) 
believed to have taken place to form the basin in which the Altona- 
Parwan lignites were deposited (Fig. 5). 
AGE OF THE NILLUMBIK PENEPLAIN 
The evidence for age is as follows: 
1. The peneplain is older than the Older Basalt which filled 
the Wurunjerri River. The Older Basalts are believed to be 
from Oligocene to Lower Miocene in age. On palaeogeographical 
grounds (opp. page), it is believed that the Older Basalt at Bal- 
combe Bay is part of the Wurunjerri flow or flows. The basalt at 
Balcombe Bay is overlain unconformably by Baleombian (Middle 
Miocene) marls. This means that the Nillumbik Peneplain is 
much older than Jutson thought (1911, pp. 477-478) when he first 
deseribed it. 
2. The peneplain is younger than the Lower Devonian marine 
sediments and the Upper Devonian igneous rocks of the Dan- 
denong Ranges, both of which are affected by the planation. Fig. 
3 shows how a ledge has been formed on the edge of the Dandenong 
lavas. 
3. When the physiography of the State is considered, and the 
peneplanations which have been effected and then raised to higher 
levels, it is clear that the Nillumbik Peneplain must have been 
formed i in Tertiary times. 
