Physiography of Werribee Area. 255 



is that the Lerderderg is not only the dominant stream of the whole- 

 Werribee basin, but is also the most ancient, and probably in- 

 herited its course from an ancestor which existed on the ancient 

 peneplan. While the long northern tributaries of the Lederderg 

 appear to have maintained their courses, and increased their ter- 

 ritory northw^ard (see Fig. 18), new post-uplift streams on the east 

 and south, with steep grades and softer rocks to help them, appear 

 to have captured part of the old Lerderderg basin. 



(b) The Brisbane Ranges. — Here again we have a huge uplifted 

 block of folded Ordovician, structurally and lithologically quite- 

 similar to the Blackwood ranges, and similarly containing a once- 

 important, centrally-situated mining field (Steiglitz). The boun- 

 daries of this block are not clearly defined, and the name is gener- 

 ally applied only to' the eastern part, the western portion being in 

 part covered by alluvium and basalt flows, cut through by the 

 deep gorge of the Moorabool River. The general level of the block 

 is about 1200 feet, sloping towards- the south and west. The north- 

 ern boundary of the ranges is the Spring Creek, while on the 

 east is the remarkably well-defined scarp of the Rowsley fault. 



Davis (ref. 16) says.- — "The simplest and most manifest ele- 

 ment of faulting along a mountain base is a straight or but mode- 

 rately curved base line, passing indifferently across or obliquely 

 along the structure of the mountain mass, which rises rather 

 abruptly and continuously on one side, while a sloping plain of 

 waste is spread out on the other." The field evidence along the 

 whole east front of the. Brisbane Ranges shows that here we have 

 a feature exactly fulfilling these conditions. This steep portion is 

 dissected by numerous tributaries of the Little River, Avhile the 

 southern half is deeply cut into by the various branches of Suther- 

 land's Creek. 



(c) The Ballan Plateau. — This structural unit is what we have so 

 far referred to as block B, or the Ballan sunkland. Although a 

 sunken area with reference to the Brisbane and Lerderderg Ranges, 

 it is relatively lifted about 1000 feet above the plains of the lower 

 Werribee — the Port Pliillip Sunkland (see block diagram. Fig. 3), 

 and since the main lines of communication between Men;ourne and 

 Ballarat give an emphasis to the sudden rise above the scarp face, 

 it is more popularly known as part of the ''Ballarat Plateau." 

 Structurally, it is of much greater coniplexity than either of the 

 two blocks already described. It would appear to have been greatly 

 faulted within its own l)oundaries, and consists of Ordovician 

 slates, etc., glacial sandstones and conglomerates, oldei- volcanics. 



