Physiography of Werrihee Area. 219 



Professor Davis in his ideal diagrams of fault-block erosion (ref. 

 16). They are most definitely observable in the field, even if only 

 noted from the plains when one travels along parallel with the 

 scarp, as may be done along the foot of the Brisbane Ranges, or 

 along the road from Bacchus Marsh to BuUengarook. 



The pliysiographie facts alone thus provide strong presumptive 

 evidence of the scarp being due to a fault. All along the base of the 

 scarp, streams have deposited an apron of alluvium. This extends 

 outward into the lower plains; it is usually over a mile in width, 

 and in places reaches outwards for greater distances. 



Geological evidence. — We may now examine the geological evi- 

 dence for further proof, and especially for indications of the 

 probable age of the fault with regard to the newer basalt. 



The general geological relations may be seen from the plans in 

 Fig. 6. We see tliat the higher blocks to the west of the scarp (A, 

 B, and C, Fig. 2) are mainly of Ordovician rocks. On the lower 

 side no Ordovician occurs close to the scarp base. In fact the only 

 Ordovician whatever on the lower block E is that along the Pyrete 

 and Djerriwarrh Creeks, and is exposed in valleys several hundred 

 feet lower than the smiliar rocks to the west of the scarp. The 

 boundary of the higher Ordovician is, mjoreover, quite sharp and 

 almost straiglit, and cuts obliquely across the strike of those 

 highly folded beds. The strike averages nearly due north, while 

 the sharp junction with the various younger beds averages about 

 15° E. of North. 



Four sections are shown in Fig. 7 (8m, 8b, 8c and 8d), drawn to scale 

 normally across the scarp where the relations can be most clearly 

 proved. It is natural of course that the lower margin sliould be in 

 many places obscured by alluvium, as shown in the geological plan. 

 The newer basalt sheet in places also partly obscures the fault. At 

 least five important sections, how^ever, may be examined where the 

 true relationships with the newer basalt are shown. Such sections 

 are especially interesting in so far as they have an important 

 bearing on tlie age of the fault. 



Section 8a shows the most northerly section tliat was critically 

 examined, at a point where the Lerderderg River emerges from the 

 ranges. It shows the glacial conglomerates let down and pre- 

 served to the east, along with middle tertiary beds, the latter being 

 capped by the basalt of the BuUengarook flow. On the summit 

 of the ranges here, small glacial remnants occur. These relations 

 provide strong confirmatory evidence of a big fault. 



