IOO 



POPULAR SCIENCE MONTHLY 



Fig. 1. Aligned Spurs, Inside Passage. Three such spurs seen on the right, the most 

 distant one showing the change in slope. Two shown on the left, with the change in slope 

 plainly visible in the more distant one. From such a condition as this there is every gradation 

 to straight walled ' canals ' Photograph by O. von Engeln. 



mountain walls, and in many places they have the characteristics of 

 grand fiords. 



Such a topography as this has, until recently, been quite generally 

 explained as a result of subsidence of the land, by which the lower 

 ends of the land valleys have been drowned by the admission of the 

 sea water into them. In this way the irregular coast of Patagonia, the 

 fiords of Norway, and other similar coast lines have been explained. 



Under ordinary conditions, the development of valleys by stream 

 erosion produces certain characteristic features which are easily recog- 

 nizable. These features are well understood by physiographers and 

 have been fully stated on many occasions, and especially by Professor 

 Davis, to whom, more than to any other, we owe our clear recognition 

 of them and their application to the problems of glacial erosion. 



One of these features is the cross-section of the valley, which varies 

 in width and steepness according to the stage of its development. A 

 young stream valley is steep-sided and gorge-like. Its width is narrow 

 in proportion to its depth. A mature valley, having long been exposed 

 to action of the weather, has been broadened out by the weathering 

 back of the valley walls so that its width is great as compared with its 



