Physiograplii/ of Yarra, etc. 491 



nong Creek and its tributaries kept their cotirses during dif- 

 ferential uplift. That being so, the upper parts of the 

 streams in the depressecl ground would reach temporary 

 base-levels, whilst the lower portions were still vertically- 

 eroding, and lateral erosion would therefore be active in the 

 upper parts, tending to form wide alluvial flats and shallow 

 open valleys. Down-stream the valleys would be more con- 

 tracted, owing to vertical erosion being for the time the nuiin 

 occupation of the streams. 



Tl)e origin of these flats* may be explained in the following way. 



Treating first of those flats near the head of the Dandenong 

 Creek, which I have referred to as the Bayswater Flats, it is 

 found that the creek here divides into several branches. The 

 main stream runs westerly from the Dandenong Ranges. A 

 branch runs southerly from Croydon, meeting the main stream 

 at Bayswater. Two other small streams run south-westerly 

 and north-westerly to the Dandenong Creek at Bayswater. 

 Each of these small streams has formed rather extensive alluvial 

 flats, but after they have joined together at Bayswater, the 

 valley becomes narrower and the wide flats are absent. It is 

 to the west of Bayswater that the higher land of the Yarra 

 Plateati occurs, and therefore the explanation that the flats 

 are due to slow and dift'erentiai uplift seems the most feasible. 



In the tributary running from Croydon, the flats are nob so 

 extensive as those more to the east. Tliis may l^e exfjlained 

 (at least in part) in the following way : — The Croydon 

 tributary is short, is in low ground, and has little drainage to 

 work upon. Hence its denudation will not be very great. The 

 main stream however of the Dandenong Creek rises in the high 

 Dandenong Ranges, where there is an abundant rainfall, steep 

 fall and fair area drained. The result is much greater for- 

 mation of alluvial flats. 



Tlie change from the flats to the more normal type in the 

 Dandenong Creek is not so marked as in the case of the Yarra. 

 The transition is more gradual. This appears to be due to a 

 more gradual change from plateau to Senkungsfeld in the 

 Dandenong Creek area. A feature that appears to corroborate 

 the explanation offered of the Bayswater Flats is the higher 

 and steeper aspect of the right bank of the Dandenong Creek 



