Physiography of Yorra, etc. 489 



The Origin of the Yarra Flats. 



These flats are part of the Croydon Senkungsfeld, and their 

 exphmation follows from the preceding discussion. Differential 

 and slow uplift, resulting in the river cutting through the 

 higher land as fast as the latter rises, means slow vertical, and 

 consequently, strong lateral, erosion in the lower ground. 

 Thus, wide shallow tributary valleys and extensive alluvial 

 flats, through which the main river meanders and builds up a 

 flood plain, are formed. 



This has apparently happened in the area in question. Dur- 

 ing the excavation of the Yering George, the river cut as fast 

 as the land rose. There has, therefore, been no time for lateral 

 erosion, and the valley here remains a gorge. In the dejoressed 

 area above the Yering Gorge, the stream was constantly reach- 

 ing a temporary base-level, waiting for the erosion of the gorge. 

 This gave great play to lateral stream erosion and atmospheric 

 denudation, with the result that the low ridges have been cut 

 into wide shallow valleys by the sluggish tributaiy streams, 

 and the Yarra itself in its meandering course has formed the 

 wide alluvial flats of Yarra Glen, Killara, etc. 



The excavation of the Warrandyte Gorge has caused the 

 same result. The distance however between the exit of the 

 Yering Gorge and the entrance to the Warrandyte Gorge is 

 short, so that the effects are on a smaller scale. The broad, 

 flat-bottomed valley of the Brushy Creek owes its characteris- 

 tic features to the same cause. 



The Antecedent Character of the Dandenong Creek 

 and Tributaries. 



The general characters of the Dandenong Creek have been 

 described above, and the slight tilting or dip to the east of the 

 Yarra Plateau in the Dandenong Creek area was referred to. 



The Dandenong Creek has generally a southerly direction, 

 but instead of running south through the lowest part of the 

 Croydon Senkungsfeld, which would appear to be its natural 

 course, it passes into the higher country to the west, but 

 works its way well into the Senkimgsfeld again near Dande- 

 none:. 



