Note 6 — III. GEOLOGY.

In some valleys the lateral tributaries enter at a higher level than the floor of the main valley, owing to the overdeepening of the latter; this overdeepening has been attributed to four different causes:—(1) Inequality in the hardness of the rocks, especially when the main valley runs along the strike; (2) the presence of joints, or other lines of weakness; (3) increased erosion of the main valley by river action owing to the elevation of the upper end of the district, while the lateral valleys were merely tilted sideways, or were protected by the presence of ice or lakes; (4) erosion of the main valley by ice more rapidly than its tributaries, owing to the ice being thicker in the former. The problem is complicated by the fact that the upper end of the main valley is frequently a ‘hanging’ valley. Any evidence bearing on the formation of these ‘hanging’ valleys should be carefully noted.