29 



III. A Prehistoric Alluvial Fan of Exceptional 

 Character at the Mouth of the Glen Osmond Gorge. 



Pis. vi. and vii. 



The piedmonts on the western side of the Mount Lofty 

 Ranges are deeply incised by numerous short streams that 

 are in a juvenile stage of development. These streams are 

 mostly working on rather steep grades, and although the 

 gathering ground is limited they exert considerable hydraulic 

 force when in flood. The Adelaide Plains, which extend 

 from the foot-hills to the sea, have been built up by alluviation 

 brought down by these streams from the hills, and within a 

 few miles from the base of the hills this transported material 

 has a thickness of 400 ft. Numerous bores on the plains 

 show that the sediments vary from a fine unctuous clay to 

 gravel of moderate size. At the exits of the numerous 

 gullies alluvial fans spread out to a greater or less extent 

 and are extended seawards along the principal channels 

 formed by flood waters. These channels are very absorbent 

 and the greater number of the streams finally disappear on 

 the plains at a lower level. 



The Glen Osmond Gorge, which opens on to the plains 

 at a distance of three and a half miles to the south-east of 

 Adelaide, has been cut through a compact quartzite, 100 

 ft. in thickness, with slaty rocks both above and below it. 

 The stream which is responsible for the excavation of the 

 gorge takes its rise on the scarp face of the foot-hills and is 

 one of the minor creeks that drain the western side of the! 

 ranges. It follows a line of faulting, with a downthrow on 

 its left bank, and is confined for some distance in a narrow 

 channel by the side of the main road. After skirting the 

 old Glen Osmond quarries near the foot of the hills, it 

 diverges from the road and follows its natural course 

 within banks from 15 ft. to 20 ft. in depth. It crosses the 

 "'Cross Roads" near Glen Osmond; passes through the 

 western portion of the public reserve (Ridge Park) ; crosses 

 the Fullarton Road on the north side of Fisher Street ; and 

 continues, confined by small culverts, through Unley, Good- 

 wood, and by a covered channel through the new Agricultural 

 Show Grounds at Keswick. It is dry during most of the 

 year and is simply a channel for flood waters after heavy 

 rains. The present size of the creek is out of all proportion 

 to the nature of the alluvial fan about to be described. 



The ground has a gentle slope, in the direction of the 

 creek, from the mouth of the gorge to the Fullarton Road, 

 a distance of about a mile. Within a restricted breadth, the 

 land adjoining the creek is more or less strewn with angular 



