Features and Growth of Lake Goongarrie. 127 



Jbetween, Cornish^ regards longitudinal ridges as due to strong 

 winds, and Free^ also points out that they seem to occur where the 

 supply of sand is small, relative to the strength of the wind. In 

 the bare open lake arms the wind blows with great force, the dif- 

 ference of strengtli of the wind in the arms and on the sand ridges 

 '(which bear '* mulga " vegetation) being very marked. As already 

 shown, the blown sand of the ridges rests on detritus due to water 

 action and gravitational drift, and this deposit, which is evidently 

 identical with and a continuation of that of the " lowlands," in 

 turn rests on the bedrock; hence the material has been available 

 to assisti in the formation of the sand ridges, and its removal from 

 its original position has helped to form, the rock-lioored arms. 



These rock floors, in ahnost every instance, rise as they 

 -approach the sand ridges; it is therefore reasonable to conclude 

 that the rock floors of the arms are not merely " resurrected " 

 .areas, but that they have suffered erosion during the formation of 

 the sand ridges. It might be objected that the sand ridges may 

 have been in existence before the arms Avere formed, and that the 

 two features are not necessarily connected, but in view of their 

 constant association and of the transition from the lowlands to the 

 vSand ridges and arms, this objection has little or no validity. 



In narrow arms the rock floor in cross section is distinctly, 

 although but slightly, concave, and in the lowest portions a few 

 inches of fine silt may occur. This silt has no doubt been laid down 

 under quiet Avater. No floAv of Avater Avas observed in any channel 

 after moderately heavy rain, but after long continued rain a dis- 

 tinct floAv AA'ould probably take place. The unfurroAA^ed rock sur- 

 face, ancl the very gentle slope to the east, together with the asso- 

 ciation of sand ridges and lake arms, hoAvever, forbid any serious 

 •erosion by Avater. It may be noted in this connection tJiat as 

 the channels or arms Aviden, the concave character of the arms in 

 • cross section usually becomes less and less until there is a level floor 

 with gently rising edges at the sand ridges. 



The rock-floored arms pass eastAvard into the rock floor of tlie 

 main lake^;, and at the eastern extremity of some of the sand 

 ridges, as Avell as at the Avestern end of some of the arms, Ioav rock 

 •<;liffs occur Avhich are in process of reduction to a rock floor, the 

 latter groAving AA^estAvard by such means. An example of this is 

 shoAvn at the eastern end of Rocky Point Island. 



1 Op. cit., pp. 292-298. 



2 U.S. Dept. of Airricultm-e. I'.iinan of Soils. P.uIIetin No. 08 (1911), p. 05. See also Blake, 

 -<i.J.f}S., voi. 53(1897), p. 229. 



y Some arms towar Is their eastern ends have floors of fine silt. 



