Physiography of Werribee Area. 279 



^'e consider the' size of the creek which carved it out. It is a' jirreat 

 irregular basin, four miles across, about eight miles in length, and 

 800 feet deep, descending sharply from the surrounding newer 

 volcanic plains. Except for the last five or six miles from the edge 

 of the scarp to the Werribee, the Parwan lies wholly in this basin. 

 It is a small stream at best, and is dry for a large part of the 

 year. The amount of arosive work accomplished is therefore all 

 the more astounding, and is indicated to some extent in plan 

 (Fig. 30), and in section (Fig. 24). 



This plan (Fig. 30) is drawn to indicate the amount and nature 

 of dissection of the fault scarp in the neighbourhood of Bacchus 

 Marsh. It also indicates the differential erosion due to differences 

 of rock structure, and this is again shown in the two sections 

 chosen (Fig. 24). The smaller stream (Parwan Creek) has carved 

 a valley of the dimensions indicated, while the much more power- 

 ful Werribee has carved out tlie well-known " Gorge." Both 

 streams are here ^exactly the same age; but while the Werribee 

 worked, as already indicated, in very resistant rocks, cutrino: 

 mainly across the strike of hard Ordivician sediments, the Par- 

 wan had the advantage of a location in level-bedded, uncompaeted 

 tertiary materials; in both cases there was a covering sheet of 

 newer volcanic. Landslips play a great part in the enlargement 

 of the Parwan basin, and their influence is seen in the steep, 

 broken, lumpy, wliite-streaked, unstable valley slopes. Residual 

 ridges and hills occur Avithin the valley, some of them capped with 

 basalt which stands at a lower level than that of the plain. Such 



B/^S/^LT SHEET 



vvvvvv\/v vvw 

 5dnd5, clc3Ly6. ere. 



Fig. 31. — Diagram to indicate the possible mode of formation of cer- 

 tain hills in the Parwan valley. 



cases possibly represent land-slipping on a large scale. A large 

 mass of ])asalt, with the thick underlying tertiaries rendered un- 

 stable by a thorough soaking during an extra-wet season, lias 

 ** slumped " lx)ldly intd the valley, as suggested by the shaded \)0V- 

 tion in Fig. 31. Tlie head waters of the Parwan and Yaloak Creeks 



•20 A 



