Physiography of Werribee Area. 297 



the writer showed nothing to suggest a marine origin for these 

 gravels, and they are no doubt part of the material carried down 

 from, the western lifted blocks, and spread out over the plain 

 before the Werribee had established its present valley. Even in 

 the much deeper deposits at the town of Werribee, the only fossils 

 foujnd are those indicating fresh water. 



^^D) Alluvial Plains. — ^^These are wholly confined to the Lower 

 Werribee, excepting those on the western part of the upland block 

 that forms the Brisbane Ranges. The latter, however, are more 

 intimatefy connected with the physiography of the Moorabool 

 basin. In the west of block E, these alluvial sheets spread out 

 from the base of the Brisbane Ranges, aaid extend southward, em- 

 bracing the You Yangs. It should some day be an irrigated area. 



The triangular patch of deep alluvial, overlying basalt, which 

 extends from the mouth of thd Werribee up to the town of the same 

 name, is an important agricultural district. This extensive irri- 

 gation settlement has suffered much in the past from the failure of 

 the main storage reservoir, but it is confidently believed that this 

 trouble is now over. Freshwater unio and fossil bones (Quarter 

 Sheet 20 N.-E.) have been found in the alluvial of this area, which 

 was doubtless formed under estuarine conditions at a compara- 

 tively recent date, when the general level of the land was somewhat 

 lower than at present. 



(iii.) Bacchus Marsh Basin. — The question of the origin of this 

 important feature is one of some difficulty. Fig. 38 has been 

 drawn to show the nature and extent of the basin. Asi we see from 

 that sketch, there is about six and a-half square miles of flat 

 country, with deep and rich alluvial soil; a more exact estimate 

 gives about 4500 acres of irrigable land. The flats are formed 

 where the most important tributaries of the Werribee all meet 

 together. Thus we have i,n our sketch the Werribee, Korkuperri- 

 mul, Lerderderg, Goodman's, Parwan, and Pyrete all assisting in 

 the building of these flats. The complex formations of the sur- 

 rounding highlands, rich! volcanics, sandstones, clays, etc., all send 

 their tribute to be blended together for the building up of the 

 wonderfully fertile soil of ''the Marsh." The greater part of the 

 work done in forming the Bacchus Marsh basin is apparently due to 

 the Lerderderg. Close to the lowest ''flats" are various slightly- 

 elevated pebbly river terraces, and as we cross the bounding line 

 between the two formations, we pass from land worth £80 an acre 

 to land worth £2 10s. or less per acre. This is especially the case 

 on the southern side of the basin, the higher ground on the western 

 slopes having somewhat better values. 



