Page:Popular Science Monthly Volume 70.djvu/104

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100
THE POPULAR SCIENCE MONTHLY.
 

PSM V70 D104 Inland passage alaska result of glacial erosion.jpg

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 recognizable. 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