253 



DESCRIPTION OF AN OLD LAKE AREA IN PEKINA CREEK, 

 AND ITS RELATION TO RECENT GEOLOGICAL CHANGES. 



By Walter Howchin, F.G.S., Lecturer in Geology and 

 Palccontology in the University of Adelaide. 



[Read July 6, 1909.] 



Plates XVII. and XVIII. 



The Pekina Creek irrigation-works are sitxiated about 

 1^ miles above the railway, which crosses the creek near 

 Orroroo. A clay dam is being constructed across the creek 

 at a spot where the rocky sides converge and form a narrow 

 gap. The height of the dam will be 70 ft., and will throw 

 the water back in a reservoir for about a mile. The clay 

 for constructing the dam is being obtained chiefly within 

 the area that will form the submerged portion of the pro- 

 posed reservoir, the excavation of which exposed the old lake 

 deposits about to be described. 



Mr. Edgar J. Bradley, the chief officer in charge of the 

 works, recognized the presence of fresh-water shells and the 

 remains of CJiara, an aquatic plant which by the secretion 

 of calcium carbonate is often contributory to the formation 

 of fresh-water limestones. I am indebted to Mr. Bradley 

 for first giving me, an early intimation of these int-eresting 

 discoveries, and also for conducting me over the ground and 

 pointing out the features of interest. 



The prehistoric lake started from the narrows, whe^e the 

 present dam is being constructed, and followed the upper 

 portions of the stream for about three-quarters of a mile, 

 and had an average width of about 7 chains. The deposits 

 were laid down on the eroded edges of the fine-grained argil- 

 laceous slates of the Tapley Hill series. These slates, with 

 the associated beds, form locally a great synclinal fold, which 

 has its eastern limb in the Mucra and Orroroo Ranges and 

 its western in the Pekina Range. The associated limestones 

 belong to the horizon of the Brighton, Reynella, and Hack- 

 ham outcrops. 



