222 



elevated rises and, at times, on the seashore these beds are 

 represented by scattered and loose stones of a cherty kind, 

 which might also have disappeared had it not been for the 

 infiltration of a siliceous cement which has made them 

 resistant to atmospheric waste. 



Like the Cambrian sediments the marine Tertiary beds 

 have been gradually reduced to a thin and almost imper- 

 ceptible covering, and even the alluvium of more recent times, 

 laid down by rivers that have long since ceased to flow, has 

 helped to fill up the hollows in the older landscape, and thus 

 contributed to the existing topographical dead level, which 

 makes of several distinct geological systems a common 

 peneplain. 



4. River Systems, Past and Present. 



In attempting to give an interpretation to the older 

 hydrographic features of Yorke Peninsula it is necessary to 

 take into account that, within comparatively recent times, 

 the two South Australian gulfs had no existence, and this 

 now almost isolated region formed part of a continuous land 

 area. The time of which we speak was probably subsequent 

 to the transgression of the sea within the limits of the rift 

 valley, in late Pliocene times, which has left its marine 

 sediments several hundreds of feet below the present level of 

 the Adelaide plains. It may also be assumed that the older 

 river system of the Peninsula dates from the time when the 

 epeirogenic uplift was in progress, and, although rising, the 

 differential movements had not proceeded so far as to bar 

 the way of the rivers of Central Australia from reaching 

 the southern coast. This conclusion is based on the facts 

 that there are important alluvial deposits on the Peninsula 

 which have no relationship to the existing lines of drainage, 

 and also that the older system of drainage ran north and 

 south, which is inconsistent with the existing coast lines. 



On account of the great development of mantle rock 

 over the country and the absence of geological sections, the 

 evidences of the past river systems are limited, and are 

 restricted, so far as known, chiefly to the eastern coast. The 

 coastal scarp near Ardrossan, which can be defined as a 

 topographical feature over a length of 2 miles, is evidently 

 " an old river terrace resting upon a Cambrian limestone floor. 

 The terrace is built up of fine to coarse river deposits, often 

 highly siliceous, which form outcrops that spread themselves 

 over the face of the scarp, equal to 100 feet in height, as 

 already described. An important river could only occur in 

 that position when the area now covered by the waters of the 

 Gulf was dry land. The same remark applies to the occur- 

 rence of similar alluvia in the cliffs and on the beach a few 



