RIVER YARRA, VICTORIA 31 
over two miles, indicating pitch. It is in this area that the higher 
levels give way to Nillumbik Peneplain levels. Also where the 
Yarra River crosses the Western Quartzites, they take a major 
change in strike, swinging round to the north-east. 
GENESIS OF THE NILLUMBIK PENEPLAIN 
As re-defined, the Nillumbik terrain is a true peneplain. It is 
remarkable that there should be so little disparity of elevation in 
the Nillumbik Peneplain in view of the enormous differences in 
rock types. Some of the summits are sandstones, some shales, 
some highly indurated quartzitie horizons, and some are soft mud- 
stones such as under the Older Basalt at Lilydale. In spite of 
these great differences in hardness, all the summits are between 
400 and 450 feet, except for the monadnock of quartzites north 
and west of Croydon. Wicklow Hill, at Croydon, reaches 650 feet, 
the height of the Yarra Plateau. 
An adequate explanation of the wide and even planation (in 
spite of variant rock types) of the Nillumbik Peneplain is called 
for, and also of the difference in level (about 200 feet) between it 
and the Yarra Plateau. Two possible explanations of the pene- 
planation suggest themselves: 
1. That the plain is one of marine denudation. This theory is 
encouraged by the relationship of this area to the sea (although 
it must be remembered that there was no Port Phillip Bay then), 
by the fact that the slopes on its seaward sides have Miocene 
marine beds on them, and that the Red Beds on the peneplain 
itself appear to be fluviatile sands and gravels spread along a 
shoreline. On this interpretation the gentle slopes on the seaward 
sides (south and west) of the peneplain would be a sloping sea- 
floor. 
Tn criticism of this interpretation, it may be pointed out that 
Richthofen, followed by many eminent geologists, has denied that 
marine planation is possible, except on a subsiding land area. 
They have claimed that ‘‘waves can cut into a still-standing land 
mass only to a very moderate extent before they will exhaust 
themselves on the shallow beach which they have carved." More 
recently, Wentworth has concluded from studies in Hawaii that 
marine erosion cannot be a factor in peneplanation. However, in 
the case of the Nillumbik Peneplain, it could be argued that it 
was a gradually subsiding land (or rising sea) that caused the 
transgression by the sea which resulted in the deposition of 
the Miocene beds. Hall (1900, p. 49) envisaged such a process 
oceurring. 
