Erosional and depositional features of river and glacier

River

Erosional: V-shaped valleys, gorges, waterfalls and rapids, potholes, meanders, river terraces. Depositional: alluvial fans, floodplains, levees, point bars, braid bars, ox-bow lakes, deltas.

Erosional work of a river is done by hydraulic action, abrasion, attrition and solution. Features:

• V-shaped valley: formed by vertical (downward) cutting in the youth stage, in the hills.
• Gorge and canyon: deep, narrow valley in hard rock, e.g. the Kali Gandaki gorge.
• Waterfall and rapids: form where hard rock lies over soft rock; a plunge pool forms at the base.
• Potholes: circular holes drilled in the bed by swirling pebbles.
• River terraces: step-like old floodplain levels left after the river cuts down again.
• Meanders and ox-bow lakes: formed by lateral (side) cutting in the old stage.
• Interlocking spurs: ridges of the valley sides that project alternately into the winding valley of a young river.

Depositional work occurs when the velocity and carrying capacity fall (on gentle slopes, at the river mouth, and on the floodplain during floods). Features:

• Alluvial fan: cone-shaped deposit where a mountain stream meets a plain, e.g. the Bhabar zone at the foot of the Siwaliks.
• Floodplain: broad, flat valley floor built by flood deposits.
• Natural levees: low ridges of sand and coarse silt built along the banks during floods.
• Braid bars and point bars: sand and gravel bars in the channel (braid bars) and on the inner bank of bends (point bars).
• Delta: fan-shaped deposit at the river mouth, e.g. the Ganges-Brahmaputra delta.
• Ox-bow lake and meander scars: cut-off bends.

Glacier

Erosional features

• Cirque (corrie): armchair-shaped hollow at the head of a glacier, made by plucking and abrasion; a lake in it is a tarn.
• Arête: knife-edge ridge between two cirques.
• Horn (pyramidal peak): sharp peak left when cirques cut back on all sides, e.g. the Matterhorn.
• U-shaped valley (glacial trough): the glacier widens and deepens a river valley.
• Hanging valley: side valley left high above the main trough; it often ends in a waterfall.
• Fjord: glacial trough drowned by the sea.
• Roche moutonnée, striations and grooves: smoothed and scratched bedrock surfaces.
• Rock basins and tarns.

Depositional features. Material deposited directly by the ice is called till (unsorted, unstratified clay, sand, pebbles and boulders); material laid down by meltwater is sorted and layered. Features:

• Moraines: lateral, medial, terminal (end) and ground moraines.
• Erratics: large boulders carried far from their source rock.
• Drumlins: smooth, elongated hills of till, shaped like an inverted spoon.
• Eskers: long, winding ridges of sand and gravel left by streams under the ice.
• Kames and outwash plains: mounds and flat areas of sorted sand and gravel built by meltwater.
• Kettle holes: depressions left where buried ice blocks melted.