AUSTRALIAN QUATERNARY CLIMATES AND MIGRATION 55 



face of the infilling basalt, and that at V, the Werribee River, 

 615 feet below it. 



It is apparent, and Brittlebank realized this, that if the rate in 

 years of the erosion of the marginal valleys could be determined, 

 it would be possible to ascertain the age of the basalt, and with it 

 the age of the gravelly clay and the age of the implements. He 

 experimented (Brittlebank, 1900) by inserting short lengths of 

 wire in holes bored in the rock at the bottom of each of the valleys 

 and recorded the times when a particular length was exposed. He 

 obtained a rate of erosion of 0-58 in. per century, which, applied 

 to the mean depth of both valleys, gives an age of over a million 

 years. Apart from the crudity of the experiment, which ignores 

 a number of factors, the main objection is that it was conducted 

 at a time when the rate of erosion was near its minimum — a short 

 lapse of time after the Postglacial Optimum. The rate of erosion 

 had consistently decreased up to that time, and its mean was 

 formerly very many times greater than the figure obtained by 

 Brittlebank. 



The basaltic lava of the Bacchus Marsh district issued either to 

 form extensive lava-fields, or scoria-cone flows that occupied pre- 

 existent valleys, at various times from the Middle Pleistocene 

 (Keble, 1946) up to a recent period in the Postglacial. Those of 

 The Island were scoria-cone flows having their main source at 

 Mt. Blackwood (Fenner, 1918). They infilled the channel PBV, 

 the middle reaches of a valley that opened, about a mile S.E. of 

 Rowsley, on to the Werribee Plains lava-field. Across this 

 opening is the Rowsley Fault trending S.S.W. Fenner (1918) 

 gives sections across the fault showing the displacement or other- 

 wise due to it. Near Dog Trap Gully, east of Rowsley, about 

 where the Island flows debouched on to the Werribee Plains lava- 

 field, he shows (sup. tit. Figs. 7, 8b). The Island scoria-eone 

 flows pouring over the scarp of the fault, but south of the Anakies 

 (sup. eit. Figs. 7, 8d) the fault displacing the lava-plains basalts. 

 He refers to a post-Newer Basalt movement and it is apparent 

 from his Figs. 7, 8d, that this occurred after the Werribee Plains 

 lava-field, but it is apparent, too, that it was before The Island 

 scoria-cone flow poured over the scarp near Rowsley. The Pleisto- 

 cene movement on the fault is newer than Middle Pleistocene and 

 The Island lava possibly late Pleistocene but probably Post- 

 glacial. 



The erosion of the marginal valleys of The Island — the Werri- 

 bee to a depth of 615 feet and Myrniong Creek to 430 feet, has 

 been taken as evidence of their great age as well as that of the 

 infilling basalt. That this is not necessarily so, is apparent when 



