Physiography of the Glenelg River. 119 



thousand feet. Some of the most famous tertiary fossil beds in 

 Australia occur in this series in the Wannon and Glenelg valleys. 



G. — The Newer Basalts. — These flows cover large areas in the 

 south-eastern portion of the Glenelg Basin ; wide level sheets with 

 occasional cones. They have considerably influenced the physio- 

 graphy, obliterating the old valley systems, diverting streams, etc. 

 The features so far developed on these areas are, of course, con- 

 siderably more recent than those of the Glenelg Basin as a whole. 



(h) Valley Types. 



(w.). — As typified in the loAver Wando River; broad and gently-^ 

 sloping in the tertiary and Jurassic, narrow and steep-sided in the 

 underlying bedrock (A). See Fig. 6. 



(x.). As typified by the Lower Wannon; broad, gently-sloping 

 valleys, with wide flats, almost wholly in tertiary and Jurassic 

 sediments. 



(y.). — As typified by Robertson's and Corea creeks; valley in 

 level coastal plain material, superimposed on hard ancient rocks 

 (A); Jurassic sediments absent. 



(z.). — As typified by the Upper Wannon (Dwyer's Main Creek, 

 etc.); deep V-shaped valleys, partly tec'tonic in origin. 



(c) The Past " Erosion Periods.'' 



We may regard the periods of rock-foi-mation (sedimentation, 

 etc.), as being " positive " periods in the geological history of the 

 area. Equally important ai-e the intervening " negative " periods 

 — the periods of rock-destruction (erosion, etc.). These great 

 erosive periods are represented only by the unconformable bound- 

 ing lines between the rocks of any two ages, as shown in Figure 6. 



(1) Post 1 Lower-Palaeozoic and Pre-lower-carboniferous. — As 

 already stated, this boundary was not investigated. 



(2) Post ? Lower-palaeozoic and Pre-jurassic.^ — This erosion period 

 ended in the formation of an area of low relief to which extended 

 reference has already lieen made. Whether lower- ? carboniferous 

 rocks (B) ever covered the more ancient rocks (A) over the whole 

 area cannot be determined. 



(3) Post-jurassic and Pre- ?older Basaltic. — Some valleys of this 

 period are preserved by remnants of basaltic flows ; such basalts are 

 older than the tertiary (Murray gulf) material, and are here loosely 

 included under the comprehensive term of " older basalts." Wliilo 

 much theory could be indulged in regarding this erosion period. 



