U-SHAPED GLACIAL VALLEYS, A MYTH?

It is classic geomorphic “wisdom” (Playfair,1802) that bedrock is eroded by streams to, form “fluvial” valleys with V-shaped cross sections. Fluvial valleys, when occupied by glaciers, are then eroded into “U-shaped” valleys. Graf (1970) found the glacial valley cross section is not really U-shaped, but parabolic, with concave slopes of the form y = ax^b.

However, this study of cross sections of recently glaciated valleys near Juneau, Alaska found only 25% of cross sections had one uniformly concave side and only .03% were concave on both sides of the valley. Of 138 slopes, only 12% were concave, while 51% were convex, and 39% of all slopes were straight.

It appears that cross sectional shape is not a reliable way to determine if a valley has been glaciated.

33% of valley cross sections had one side less than 2/3 as high as the opposite. This indicates that glacial flows in these valleys were most likely constrained and directed by adjacent glaciers rather than by the bedrock.