^55 



3. A deposit of fine silt and clay was laid down by the 



stream in this area of arrested drainage until it 

 reached a thickness of 50 ft. The great length 

 of time that elapsed during this accumulation of 

 sediment is evident, both from the fine lamina- 

 tion of the beds and the great thickness of Ghara 

 deposits, including the thin layers of fresh-water 

 limestone. 



4. An increase of the angle of grade rejuvenated the 



stream, and thereby increased its erosive power, 

 by which the lake became drained ; the stream 

 cut down through the lacustrine deposits and into 

 its rocky bed 2 ft. below its former level. 



The origin, growth, and extinction of this small river 

 lake offer several points for investigation of more than ordin- 

 ary int-erest. 



Mr. Bradley suggested to me the possibility of a land- 

 slip having occurred in the creek, blocking the channel and 

 for a time damming the waters back, forcing the stream to 

 take a new course over the shoulder of the spur (marked h 

 in fig. 1), where a bed of gravel occupies a slight depression 

 in the ground. This is a perfectly legitimate explanation 

 and an event very likely to occur in a narrow gorge, but the 

 surrounding circumstances do not seem to favour such an ex- 

 planation in this case. The mass of material required to 

 form such a dam would have to be very great, as 50 ft. of 

 sediment has been laid down in the backwaters area. More- 

 over, the arrested current would speedily rise to the height 

 of the barrier, giving a depth far too great to permit of the 

 growth of thick forests of Ghara, and the fineness of the sedi- 

 ment indicates the absence of torrential action. 



Lakes arise from various causes. They mostly occur in 

 established lines of drainage, and arise from the develop- 

 ment of some physical barrier, which gives a temporary check 

 to the drainage, as, for example, in the movements of land 

 ice or by the oscillations of the earth's surface. It is the last- 

 named cause which I think has been primarily responsible for 

 the origin of the Pekina Creek lake. 



The hydrographical features of the Orroroo district are 

 certainly very remarkable. There is a wide valley, or rather 

 plain, bounded by distant ranges of hills, and containing 

 within its area isolated hills or groups of hills. The drainage 

 of this area consists of a great number of streams and stream- 

 lets that have no relationship to each other. There is no 

 trunk river, and the drainage is broken up into isolat-ed 

 fragments. The valley is there, but the river is wan.ting. 



