Sedimentary Rocks. 45- 



the pre-Newer Volcanic streams trended southerly, and were 

 divided by north and south water partings. 



Following the main stream — Jackson's Creek — northwards- 

 from Sunbury, we find that it has cut down on to Upper Ordo- 

 vician shales and Riddell Grits. From Lancefield Junction west- 

 wards there are alternating outcrops of Newer Basalt and 

 palaeozoic rocks — Upper Ordovician and Riddell Grits, the basalt 

 representing the old valleys and the sediments the intervening 

 watersheds. There is evidence that the older valleys were in 

 a mature state, and pre-Newer Volcanic gravel's rest on their 

 slopes in many places. Some of the earliei; streams would natur- 

 ally be larger than others, and in particular two large streams 

 indicated- by the wider stretch of basalt seem to have converged 

 from N.E. and N.W. to a confluence near Mount Aitken, and 

 then to have •trended south between Mount Tophet and the 

 Western Kororoit Creek. (See Fig. 2.) 



Proceeding west to the Djerriwarrh Fault, the most important- 

 change, the reversal of the Upper Bullengarook to form por- 

 tion of Jackson's Creek, has already been referred to. The 

 waters of this reversed stream, and of its original source north. 

 of Gisborne R.S., would flow to the south through the Mount 

 Aitken Gap. The Upper Pyrete, with its boat hook tributaries, 

 probably flowed north at this period, and added its waters to the 

 same stream. The main laterals of the Bullengarook lava field 

 were Goodman's Creek and Cataract Gully. Goodman's Creek 

 had scope for development, flowing over homogeneous rocks, but 

 Cataract Gully found its activity limited by the resistent basalts 

 of Mount Bullengarook and Hare's Hill. An eastern tributary 

 better situated for erosion developed into the main stream — 

 now the south-west reach of the Pyrete. This diverted the ljead 

 waters of the Glendoon Creek from the Djerriwarrh system, 

 taking from it the reversed drainage which the Glendoon had 

 captured from Jackson's Creek. The history of the Upper 

 Pyrete and its tributaries would thus be : — 



(a) They were formed by drainage flowing north to 

 Jackson's Creek consequent on the reversal of the 

 direction of the latter by erosion westward form the 

 Djerriwarrh Fault. 



(b) They were captured by the Glendoon Creek, most 

 likely after the volcanic outbursts, the southern grade 

 being much steeper than the northern, partly owing 

 to rejuvenation consequent on an east and west fault, 

 through Coimadai. 



