298 E. C. ANDREWS. 



On the Ice Flood hypothesis the deep rock basins and the 

 lakes impounded by moraines should be filled with alluvium 

 by the dying glaciers and by the ever increasing water 

 action. And this condition of things it is which obtains 

 there at present. From the steep valley sides and along 

 the main channel itself debris is being carried to build out 

 alluvial fans from the lake heads and sides into the basins. 

 At an early stage the finer silt will have filled up the 

 deepest basin hollows, even those which occur in the upper 

 valley rock basins; while lower downstream, owing to 

 greatly increased water volume and gathering grounds for 

 debris, the rock basin will have been completely alluviated, 

 and converted into a meadow traversed by a meandering 

 stream. Thus at the present time in the Sierras, it would 

 be impossible to say, by merely sounding the lakes just 

 where the deepest points of the rock basins were originally 

 located. From the high vantage point of the summit of 

 Mount Darwin the writer has noted the wonderful silting 

 up action which has taken place even in the BvolutionValley 

 lakes, which from the sides of the rock basins themselves 

 appear to be free from silt. 



This being so, it might be asked how we know that the 

 recent glacier floods have corraded most deeply along cut- 

 ing curves and how we know such to be similarly situated 

 to the deepest points of cutting curves formed by other 

 stream flood action, seeing that no lake nor fiord basin is 

 known to day, that is not silted up partially, or that does 

 not possess an alluviated head. 



The case is an easy one. The geometry of the " gravi- 

 tative curve" is simple, and when the greater portion of 

 the profiles remain intact, the remainder can be fairly easily 

 restored, for from the mechanics of stream action we know 

 the peculiar basin types to be expected. If thus we find a 

 profound canon whose opposing walls are not set far apart 





