EROSION AND ITS SIGNIFICANCE. 125 



glacial floor from considerations of volume and friction, but 

 will excavate a somewhat similarly shaped channel in its 

 floor as opportunity offers. 



Depths of canons in plateaus. — If one knew the details 

 of stream corrasion well, then having given the volume, 

 the load, the steepness of the thalweg, and the strength of 

 the rock structures acted upon, one could tell to what 

 exact depth the streams could cut their bases into any 

 plateau. For it is evident that the channel bases will be 

 cut down towards base level so long as the stream is 

 enabled to carry its load as a whole over its channel struc- 

 tures. In the absence of such exact knowledge it may be 

 helpful to describe several canon types: — 



In Eastern Australia a peneplain has been raised in lat& 

 geological time to form a high plateau. In this surface 

 streams such as the Shoalhaven, Hawkesbury, Olarence- 

 and Barron have cut gorges, and they may be seen to be 

 still growing by head ward recession. 



TheTallong Plateau through which the Shoalhaven flows 

 is 2,000 feet in height, and 50 miles from the sea the gorge 

 of the stream is 1,600 feet in depth. 



The Wollondilly, at a distance of 200 miles from the sea 

 flows in a gorge through a plateau 2,600 feet high, yet its 

 base is only 600 feet above sea level at this point. 



The Barron is an extremely short river possibly not more 

 than 60 or 70 miles in length. It has recessed its front for 

 a distance of about 10 or 15 miles. The base of the gorge 

 under the giant waterfall (750 feet) where it leaves the 

 upland is said to be only about 200 feet above sea level. 

 Similarly for other streams of Eastern Australia. 



The Merced Gorge in California flows through a lofty 

 plateau for many miles, nevertheless at El Portal, its base 

 is only 1,850 feet above the sea. 



