808 THE GEOGRAPHICAL SIGNIFICANCE OF FLOODS, 



basins and broadly U-shaped contours are, in each case, found 

 both similarly shaped and similarl}' situated. 



Chamberlin and Salisbury (4,/.p.79) mention the occurrence of 

 a basin at Fort Jackson, excavated by the Mississippi, 250 feet 

 below baselevel. The authors do not state the occasion, or 

 occasions, necessitating such powerful gravitative thrusts; 

 whether obstructions, convergence of tributaries, or formation of 

 small basin at couvergence and later enlargement linearly bv 

 head ward growth, 



In ordinary stream-valleys also, freedom from debris often 

 characterises channel-narrows, while aggradation marks channel- 

 divergences. 



In the discussion it will be seen how applicable all this 

 is to the case of glaciation, whether considered as Alpine or 

 continental glaciation. 



•i. Glacial valley -co nto^irs. — The magnificent descriptions of 

 Davis, Gannett, Gilbert, Johnson, McGee, Penck and others, 

 have made typical glacial valley-contours familiar to all readers. 



In New Zealand one finds equally characteristic contours in 

 Hall's Arm, Crooked Arm, Lake Te Anau, Milford Sound, and 

 the Hollyford Yalley. 



The canon floor is frequently terraced (19,2-2n), the terraces 

 being subhorizontal and rising from one to the other by means of 

 amphitheatres or cirque-like forms. For considerable distances 

 spurless walls characterise the canons, while magniticent cirques 

 commonly form valley-heads. Small rock-basins lie at the feet of 

 the cirques, while frequently low cols, U-shaped in section, lead 

 to similar valleys across the divides. 



The canons of south-western New Zealand end in large rock- 

 basins, sometimes 40 to 50 miles in length, their bases being as 

 much as 2,000 feet below local or main baselevel. Moreover, 

 these basins show reversed grades lower downstream, while 

 tributary streams are well hung up above the main lake or sound 

 floors. The following sketches, as also Plates xliv. and xlv., illus- 

 trate well these contours of formerlv glaciated regions. 



