22 RIVER YARRA, VICTORIA 
to the strike of the basement rocks, which consist of a series 
of marine sediments—mudstones, sandstones, and shales. The 
physiography of the Upper Yarra is essentially youthful. There 
are many rapids, and terraces of torrent gravels are in evidence 
in a number of places. 
The second part of the Upper Yarra from the McMahon Gorge 
to the Warburton Gorge owes its most characteristic features to 
control by the latter gorge. As Edwards (1932) has described, 
daeitie lavas, a granodiorite intrusion, hornfels, and a group of 
acid dykes stand athwart the course of the river, resulting in the 
formation of a gorge, and the partial reduction of the country 
upstream from it. Big Pats Creek is a tributary which enters 
just upstream from the gorge. It is a stream marginal to the 
granodiorite intrusion and its metamorphic aureole of hornfels. 
MIDDLE YARRA 
When the river emerges from the Warburton Gorge at Mill- 
grove, it assumes a quite different character, and owes its form to 
different causes. It is for this reason that this section of the river 
is placed in a different category and called the Middle Yarra. The 
river flows through a mature valley with a wide flood plain. It 
receives at Yarra Junction the waters of the Little Yarra, at 
Launching Place those of the Don River, and at Healesville those 
of the Watts River. To the north the valley is walled by the 
igneous complex of Ben Cairn, and the Tool-be-wong granodiorite 
intrusion. To the south, granite and granodiorite outcrop (Baker, 
Gordon, and Rowe 1949). Obviously these rocks have played a 
major part in determining the direction of drainage in this 
area. In fact, the voleanie rocks of the Dandenong Ranges, and 
a series of granite and granodiorite intrusions to the east of it, 
are responsible for the westerly flow of the Yarra in its upper 
reaches (Fig. 2). 
At Woori Yallock, the river changes course to a more or less 
northerly direction which is maintained for eight miles (measured 
in a direct line) to Healesville. The Woori Yallock Creek, which 
has a northerly course, flows more or less parallel with the Yarra 
from near Woori Yallock to its junction with the main stream 
north of Killara, three miles away. This is to be explained by the 
controlling effect of the nature and strike of the bedrock. Indeed, 
the country rock is the major factor in determining the course of 
the Middle Yarra between Woori Yallock and the Warrandyte 
Gorge. In early Tertiary times, when the present Yarra drainage 
system in this area was initiated, the extent of the Older Basalt 
lava field was the main factor. This, in turn, was determined by 
