Physiography of Bulla Area. 325 



Physiography. 



General Stirvep.— The dominating feature of the area is the 

 •great basalt sheet which slopes gently from the north-west to the 

 south-east, with a slope of 60 ft. per mile, descending from 600 

 ft. to 300 ft. in five miles. The Deep Creek, Jackson's Creek, and 

 the Maribyrnong River have entrenched themselves in this low 

 plateau to a depth of 300 ft., and they are vigorous yonng 

 streams, cutting deeper and deeper into a plain that is also very 

 youthful in character. Two granodiorite masses rise 100 feet 

 -vabove the lava surface. 



Meanders. — Deep Creek, Jackson's Creek, and the Maribyrnong 

 ;River meander in a trench about 300 ft. in depth. These streams 

 originally flowed on the surface of the basalt plain, and the slight 

 curves in their old courses became more and more pronounced as 

 rthe streams deepened their beds. While lateral erosion was at 

 work deepening the curves, vertical corrosion was deepening the 

 valley, and this combined action has resulted in an alternating 

 series of spurs and river cliffs along each stream. Waterworn 

 pebbles of basalt, quartz, quartzite, etc., along each spur, afford 

 evidence of the former position of the stream. Ihis type of 

 .meander is in sharp contrast to the flood-plain meander, where 

 -only lateral erosion is active. 



Several writers (1) describe an entrenched meander as one where 

 the original meander has been preserved, and where the opposite 

 banks of the stream make approximately equal angles with the 

 surface of the ground. According to these geographers, the spurred 

 meanders of Bulla and Sydenham would not be entrenched mean- 

 ^ders. W. M. Davis, however, refers to the spurred character of the 

 entrenched meanders of the Meuse and Seine. (2b) It is impor- 

 tant to note that the present meanders of Deep Creek are not 

 simply the preserved meanders of the old stream, as indicated by 

 .J. W. Gregory (3). The length of the present curves is very much 

 greater than that of the old curves, oAving to the lateral swinging 

 of the streams, but the radii of the curves have remained approxi- 

 :mately the same, (2a). 



Down-valley Sireej) of Meanders. — This sweeping movement is 

 not so pronounced as in streams flowing through soft rock, but tlie 

 effect can be definitely seen in most of the spurs along either creek 

 and in the position of the small flood plains. The spurs are not 

 fgvmmetrical in section, the steeper side always pointing up- 



