286 Charles Fenner : 



Ktructiiig basalt bar is very noticeable in the grades of the 

 Myrniong — above that bar the fall is 100 feet to the mile, below it 

 is over 400 feet to the mile. 



(f) Korkuperrimul Creek. — The upper part of this creek is 

 exactly similar in age and origin to the Myrniong Creek. It flows 

 south for about three miles in Ordovician, with basalt on the 

 right, and high Ordovician on the left. It then turns south- 

 easterly, possibly following the line of the Greendale fault. Into 

 a let-down block of glacial south of this line, the Korkuperrimul 

 then turns its course, and so in a gradually widening valley, and 

 with a gradually lessening grade, it passes through glacial sand- 

 stones, older volcanics, and tertiary beds to the Werribee. 



That part of the Korkuperrimul that lies wholly in the older 

 basalt is quite steep sided; the characteristic contours are repre- 

 sented in Fig. 10. 



The Korkuperrimul, as elsewhere stated, has captured some of 

 the Lerderderg territory. Morton's Creek, flowing in a wide val- 

 ley through older basalts, and entering the Korkuperrimul on its 

 right bank, is the chief tributary. The last mile of the Korkuper- 

 rimul, past Bald Hill, is approximately along the line of the 

 RoAvsley Fault, but that feature is now cjuite obliterated in the 

 extensive erosion of the soft rocks there (See Fig. 30.) 



(g) Goodman^ s Creek. — This long and important south-flowing 

 tributary of the Lerderderg must, from the point of view of 

 origin, be considered in conjunction with its *' twin stream " — the 

 Pyrete Creek. The area now drained by these two streams was 

 originally the territory of the ancient Bullengarook Creek. When 

 the latter stream was filled by basalt, the two creeks under discus- 

 sion arose, one on either side. There they have entrenched them- 

 selves in deep valleys, with the lava tongue now standi rig high up 

 between them. 



Fig. 16 depicts the origin of such a pair of streams; the facts 

 are of course well-known, but the diagram is intended to give 

 definiteness to the idea. A shows a stream, with a cross-section of 

 its valley. B represents this stream valley, its lower parts filled 

 by a lava flow. In C two "twin streams" have commenced their 

 work, and a further stage is shown in D. It will be noted that 

 both valleys must typically receive their longest tributaries on 

 the sides remote from the flow; this is very distinctly and charac- 

 teristically seen in the case of Goodman's and Pyrete Creeks. In 

 some cases of course, one stream may for some reason desert the 

 side of the flow, as the Korkuperrimul has done; or one stream 



