Physiography of Werribee Area. 245 



troughs that we may reasonably believe to have been formed at the 

 same period. The idea is diagrammatically illustrated in Fig. 14. 



(c) 27ie Netaer Volcanic Sheet and its Effects. 



Fig. 15 has been drawn to show the extent to which the newer 

 basalt has aliected the physiographic features of this area. This 

 map of course show^s the minimum extent of the basaltic sheet — it 

 has in many parts been hidden by later alluvial deposits, while in 

 other places it has been eroded away. With the exception of the 

 high block of the Blackwood and Lerderderg Ranges, with a small 

 portion of the Gisborne highlands, and most of the Brisbane 

 Ranges, the basalt sheet must have practically covered the whole of 

 the area. The Blackwood Ranges seem to have escaped on account 

 of their being then much higher than the general low even surface 

 of the remainder of the area. Even so we have various small areas 

 of lava on this block, with evidence that these patches were once 

 more extensive — as at South Bullarto, Wuid Kruirk, Mt. Wilson, 

 Blakeville. Upper Werribee, Green Hills and Mt. Blackwood itself. 



It will be seen that the task of deciphering the buried physio- 

 graphy is a very difficult one. The well-known figure of speech 

 comparing such a task with the deciphering of a palimpsest is par- 

 ticularly applicable. The old stream courses have been almost 

 entirely obliterated, and only at rare intervals do w^e dis- 

 cover relics, mainly where the post-basaltic streams have cut 

 through the volcanic sheet, exposing something of the rocks below. 

 To use the words of one geological writer (ref. 32) : — '' Like a moss- 

 grown inscription upon a slab of marble, though veiled it may be 

 deciphered." The writer must confess that many of the problems 

 concerning the buried rivers remain unread, but some important 

 evidence has been collected. 



It may be first stated that the surface before the newer basalt 

 flows was of low relief, with one or tw^o exceptions. The Greendale 

 and probably the Gisborne scarps were in existence, as were the 

 wide valleys of the Lerderderg and Gisborne creek. One or two 

 monadnocks, as the You Yangs and the much lower granitic portion 

 of the Anakies stood above the general level, as also did a low 

 rounded dome of Ordovician slates and granite in the neighbour- 

 hood of our present W^erribee gorge. Elsewhere the relief was not 

 marked, since apparently blocks B, C, and E must have been almost 

 at sea level. In the raised blocks k and D, as mentioned, the relief 

 was much more distinct, the dissection of the lifted peneplain in 

 those areas having by this time been well started. Mr. Charles C. 



18 



