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Page:Popular Science Monthly Volume 70.djvu/107

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103
GLACIAL EROSION IN ALASKA

discordance in the other directions mentioned above, calls for explanation.

The partial submergence of a region traversed by a series of valleys with the characteristics just stated, would produce results which can be readily and accurately predicted. The line up to which the new sea level reached would be rendered irregular for two reasons. In the first place, the overlapping spurs would introduce a winding coast line in the fiords, with capes on one side opposite reentrants on the other. In the second place, since the tributary valleys joined the main valleys at grade, the sea water would enter their mouths and thus transform their lower portions to bays.

Examining the actual conditions along the Inside Passage to Alaska, we find very wide departures from this postulated result of a drowning of normal land valleys. Many of the passages are in the form of long straight 'Peaches' and 'Canals,' up and down which one can look for miles without obstruction to the view. In other cases the 'Reaches,' though not perfectly straight, have alternating projections and reentrants (Fig. 1). These, however, depart from typical overlapping spurs in two important respects. In the first place, they are much less pronounced. In the second place, instead of having a uniform slope from the crest to the tip of the spurs, they have a moderate slope above, like that of ordinary valley spurs, but terminate on the water side in a steep and even precipitous slope. They have the appearance, therefore, of being truncated valley spurs; and a view through such a channel often shows a succession of these partial spurs with the truncated faces in alignment. The general appearance of these aligned spurs suggests that some powerful rasping agent has moved through the fiord and truncated the overlapping spurs back to a fairly uniform distance.

The fiords of the Inside Passage furnish all gradations from typical overlapping spurs to aligned spurs, and to straight, smoothed 'Canals' from which all semblance of spurs has been erased. In the latter case the valley walls themselves often possess a double slope, steep and even precipitous below, more gentle above. The steepened lower slope has the appearance of having been incised in a valley whose remnant is represented by the upper more gentle slope.

In those 'Peaches' which are long and straight, and in those with aligned spurs, the tributary valleys enter the main valley at very different levels. Some, especially the larger, enter below the level of the sea, and in these cases there are bays in their mouths (Fig. 2); many others have their mouths high above the fiord level (Figs. 3, 4 and 5). Although there is no uniform height at which these side valleys enter the main trough, in general it is true that, the smaller the tributary valley, the higher its mouth lies above the main valley bottom. These are called hanging valleys because their mouths hang above the bottom