Geology of Broadmeadotvs. 157 



The township of Broadmeadows lies sheltered in the valley of 

 the Moonee Ponds Creek, immediately above the confluence of 

 the right and left branches. Below the township, the valley is 

 walled on both sides by new^er basaltic plains, where, with a 

 depth of 150 feet and an average wddth of 25 chains, the sides 

 have an average slope of 1 in 4. Here, then, the valley is geo- 

 logically young, and similar to all others which have been carved 

 out on basalt plains. The youthful character is emphasised by 

 the steep grade of the tributary creeks. The small tributary in 

 Se-tion VI., Will Will Rook, with a length of little more than 

 half a mile, has an average slope of 1 in 20. 



Above the township, where the two stream courses mark the 

 junction of the newer basalt and the palaeozoic sediments, the 

 valley presents a little more mature appearance. Lateral erosion 

 has proceeded at a more rapid rate in the sediments unprotected 

 by a basalt cap. 



The present valley of Broadmeadows is post newer basaltic. 

 The basalt streams, which probably came from the north (1) 

 flowed round the granite hill called Gellibrand's Hill, and prob- 

 ably never covered the whole of the area now mapped as silurian. 

 In the development of the new^ drainage sj^stem consequent on 

 the lava flows, the streams, here as elsewhere, have found their 

 easiest course along the junction of the basalt with the older 

 rocks. So we find the upper part of the stream courses prac- 

 tically outlining the boundaries of the newer basalt with the 

 granite, with the older basalt and with the silurian rocks. Of 

 these the silurian sediments are least resistant to denuding 

 agencies, and hence, while we find the granitic hill still main- 

 taining its relatively high altitude, the old silurian hill, w^hich 

 banked up against the granite, has been very largely worn away 

 and now forms part of the hollow^ in w^hich the town of Broad- 

 meadows is situated. The thin skin of basalt which consoli- 

 dated on the lower southern slopes of this old hill is also partly 

 worn away, as is evidenced by the presence of a small oval- 

 shape outlier of newer basalt within the fork of the tw^o 

 branches of the Moonee Ponds Creek. This suggests that the 

 course of the creek onw^ards towards the south has been deter- 

 mined by a southern continuation of this old silurian ridge from 

 Gellibrand's Hill. 



