The Valley Sides Look Sliced Off. Something Very Large Went Through
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Ridges that once projected into a winding river valley end abruptly in steep faces because a glacier could not bend around them and removed their ends instead.
What was there before
A river cuts a valley by eroding downwards and follows the easiest route, so it winds around obstacles. The ridges of higher ground between tributary valleys therefore project alternately from each side into the main valley, and the river threads between them. Standing in such a valley, the view along it is closed off by these interlocking ridges, and you cannot see far in either direction. That arrangement is characteristic of a valley shaped by water and is what existed in most glaciated uplands before the ice arrived.
What the ice did to them
A glacier behaves quite differently from a river:
- •It fills the valley rather than running along the bottom
- •It is too thick and too stiff to wind around obstacles
- •So it removes whatever projects into its path
- •Erosion acts on the valley sides as well as the floor
- •The result is a straighter valley with a flatter floor
- •And ridge ends cut off as steep faces
How to recognise the result
The evidence is unmistakable once the before and after are held in mind. Ridges descend from the upland and stop abruptly at a steep, frequently cliff-like face rather than tapering gradually to the valley floor. The faces on both sides line up along the valley, marking where the ice edge ran. The valley itself is straight enough to see along for a considerable distance, which is impossible in a river valley of the same size. And the cross section is a broad trough rather than a narrow notch, with steep sides and a flat floor.
Why the valley is a trough
The characteristic cross section follows from how ice erodes compared with water. A river erodes only where it touches, which is a narrow strip at the bottom, so it cuts a narrow notch and the sides are shaped afterwards by weathering and slumping. A glacier fills the whole valley, so it erodes the sides as well as the floor, widening as it deepens. The result approximates a broad trough rather than a notch, and the sides are steepened rather than graded. The shape is frequently described as a letter U against the river valley's V, which is a rough approximation rather than a precise one.
What else came with them
The same passage of ice leaves a set of associated features and reading them together is how a landscape is dated. Tributary valleys that were not deepened by the main glacier are left high above the floor, so their streams enter over a waterfall. Bowl shaped hollows at the valley head mark where the ice accumulated. Sharp ridges separate adjacent hollows. Scratches and polished surfaces mark the rock at the sides. Deposits of unsorted material lie along the floor and across it. A valley showing all of these was occupied by ice rather than merely by cold conditions.
The takeaway
A river winds between interlocking ridges that project alternately from each side, and a glacier filling the same valley is too stiff to bend, so it cuts the ends off them and leaves steep faces lined up along a straightened trough. Hanging tributary valleys, bowl shaped heads, sharp dividing ridges and unsorted deposits accompany them, and together they indicate ice rather than cold.