Rivers carve valleys shaped like a V, glaciers grind them into a U
You can often tell what made a valley just by its cross-section. Running water saws narrow V-shaped notches; moving ice scours wide U-shaped troughs with steep walls and flat floors. Tributary glaciers leave side valleys hanging high above the main one, which is why so many alpine waterfalls plunge off cliffs.
A valley is a long, low trough sloping one way along its length, usually carrying a river. Most are carved by streams over immense spans of time through abrasion, pot-holing, chemical corrosion and weathering. Small valleys feed bigger ones in a branching network that eventually reaches the sea or an inland basin. Some valleys are carved by glaciers, and rift valleys, like the Albertine and Gregory rifts, form mainly as the crust pulls apart rather than by erosion.
A river valley changes character downstream. High up, the stream cuts downward into a steep V, often twisting between interlocking spurs where hard rock or faults steer it. In the middle reaches, merged streams widen the valley, eroding the outside of bends and depositing on the inside, so the river starts to meander. Near the mouth, gentle gradients let deposition win and a broad floodplain forms. Rapid down-cutting, triggered for example by uplift or a drop in sea level during an ice age, can produce gorges, canyons and river terraces.
Glacial troughs begin in cirques, armchair-shaped hollows where ice first gathers, then extend down valleys previously shaped by water. Rock frozen into the ice abrades the valley into a parabola that minimises friction, and it truncates old spurs. Basins gouged into the floor may later hold ribbon lakes or, on coasts, become fjords. A little stream wandering through an oversized trough is called a misfit. Tunnel valleys cut by meltwater beneath ice sheets can be 100 kilometres long, 4 wide and 400 metres deep.
Hanging valleys form where a small tributary glacier carved a shallower trough than the main one, leaving its mouth stranded high above; different erosion rates in harder rock can produce the same effect without ice. They exist underwater too: a branch of Norway's Sognefjord called Fjærlandsfjord is about 400 metres deep at its mouth, beside a main fjord of 1,200 metres. In the Tyrolean Inn valley, the shoulders sit only 100 to 200 metres above the floor, and villages there bask in winter sun while fog fills the valley below.
Source: Valley
