Physiography of Werribee Area. 299 



end of the depression, through a gorge so narrow and deep that it 

 cannot be availed of for any line of communication (see Fig. 38). 



That this basin has been carved out l>y the rivers mentioned 

 above is tjevond dispute. It is easy to understand how, once the 

 basin was commenced, the rivers would gradually extend it further 

 outwards and down-stream, by "side swinging." The commence- 

 ment of the basin was probably made by the Lerderderg and Wer- 

 ribee at their point of junction below the fault scarp. That junc- 

 tion must then have been at a point somewhere west of the town of 

 Bacchus Marsh, and at a higher level than the present flats. 



Hart (ref. 22), after discussing the erosion of the Parwan 

 valley, says : ''A similar explanation can be applied to Bacchus 

 Marsh itself." It appears to the writer that there are important 

 differences in origin between the Parwan Basin and the Bacchus 

 Marsh basin, although the rocks worked in are closely similar. 

 The Parwan Basin is on the up-throw side of the fault line, 

 with the accompanying rejuvenation of its streams. The Bacchus 

 Marsh basin lies on the down-throw side of the fault, where 

 aggradational Avork would be done by the rivers, possibly until 

 such time as the Lower Werribee established a channel in the newer 

 basalt. The ParAvan basin has been mainly accomplished by the 

 headword erosion of steep tributary valleys. This can hardly have 

 been the case with the Bacchus Marsh basin. 



If the newer basalt covered the Bacchus Marsh area right up to 

 the scarp, it is perhaps less easy to conceive how the start was 

 made towards the formation of the basin and the flats. The pre- 

 liminary factors that led to the origin of tlie Bacchus Marsh Basin 

 may be stated thus : — 



(1) The probable meeting near to and south of Bachus Marsh 



of at least two impoi-tant pre-basaltic streams, the 

 " ancient Werribee " and tlie " ancient BuUengarook." 



(2) The filling of these valleys with newer basalts, which also 



spread out, covering mucli of tlia adjacent country, and 

 oveidyiiig easily-eroded, level-bedded sands ^nd clays. 



(3) The meeting together of several important streams, on this 



a lea. 

 As the rivers, above the scarp, carved fairly deep gorges (see 

 Fig. 30), they would also cut valleys through their own alluvial 

 deposits at the base of the scarp, and, later, into the rocks below. 

 Since the only couise for these rivers was aci'oss the volcanic 

 plains to the south-east, a channel was developed there in the 

 basalts. As the lower part of the stream gradually deepened 



