258 



ing Orroroo station, going north, passes by a viadixct over 

 the Pekina Creek and then through a cutting of this high- 

 level alluvial. The Orroroo bore on the plain is 1,287 ft.'i) 

 above sea-level. The Orroroo railway station, situated on 

 one of the old river terraces, is 1,380 ft. above sea-level, or 

 93 ft. above the bore, and the surface level of the prehistoric 

 lake is 1,460 ft. above sea-level or 173 ft. above the bore. 

 The lake-level must at one time have closely approximated 

 to the main valley-level, along which the trunk river pur- 

 sued a sluggish course. It was this high level of drainage — 

 150 ft. or more above the present plain — which dammed back 

 the waters of Pekina Creek and led to the formation of the 

 lake above the narrows in the stream. 



There has, therefore, been a lowering of the Orroroo 

 and Walloway Plain, to the extent of at least 150 ft., since 

 the time when the Pekina Creek was unable to transport its 

 load in consequence of the lowness of the grade. The agent 

 of erosion and transport by which the main valley became 

 lowered was probably wind. The soil of the plain is a very 

 fine silt, and in the summer-time the plain is constantly 

 swept by duststorms.<2) This reduces the general level, leav- 

 ing low hills of sand and gravel along the edges of the plain 

 50 ft. or 60 ft. in height. Passing wp the Pekina Creek 

 the alluvial not only caps the banks on either side, but has 

 choked the old tributaries of the creek with accumulations 

 of clay and gravel, and over these thick deposits of gravel 

 waterfalls occur, the streams not having siicceeded in the in- 

 terval of re-excavating their beds to their former level. The 

 elevation of the country having resulted in parting the drain- 

 age and wind-waste lowering the plain, a better grade was 

 establislied, and the lateral creeks being rejuvenated thereby 

 have incised their .aggraded beds. 



If our theory be correct, then at the time when the now 

 extinct lake was created, the Pekina stream was practically 

 on a level with the main river of the plains. The whole of 

 the present gorge (except the excess of erosion which may 

 have taken place subsequent to the laying down of the 

 lacustrine deposits) was filled up with stream-Avash. The cur- 

 rents would become increasingly weak and sluggish as the 

 declivity was lessened, and the stream would widen out into 

 marsh. At the site of the lake thei'e are extensivi flats be- 



(1) These figures, kindly supplied by Mr. Bradley, are the 

 latest determinations. 



(2) Mr. Bradley informed me that ■vvliir]\vinds were of daily 

 occurrence on tlie plain during tlie summer. Whilst engaged 

 surveying on tlie plain he lias counted as many as twenty-six 

 whirlwinds in one day. 



