Glacial and glacio-fluvial deposition

Detail

1. Two kinds of glacial deposit

• Glacial deposition: a glacier picks up rock debris as it moves. It drops this load when the ice melts or slows down. The debris can be dropped in two ways:
• straight from the melting ice, which gives glacial till

• by meltwater streams, which gives outwash (glacio-fluvial deposits: "glacio" means ice and "fluvial" means river)

• Glacial till: unsorted coarse and fine debris (big boulders and fine clay mixed together), dropped directly by melting ice. The fragments are mostly angular to sub-angular, with sharp edges, because no water rolled them around.

• Outwash deposit: debris carried by meltwater streams that flow out from the bottom, sides or snout of a glacier.
• Water sorts the load by size and lays it in layers, so it is roughly sorted and stratified (stratified: laid down in layers).
• Rolling in water knocks off the corners, so the fragments are somewhat rounded.

Feature	Glacial till	Outwash deposit	River alluvium
Laid by	Melting ice directly	Meltwater streams	Rivers
Sorting	Unsorted	Roughly sorted and stratified	Well sorted and stratified
Fragment shape	Angular to sub-angular	Somewhat rounded	Rounded
Typical landforms	Moraines, drumlins	Eskers, outwash plains, kames	Floodplains, deltas

• Exam trap: till and river alluvium. River alluvium, like outwash, is sorted and rounded by water. Till is neither. A mixed heap of angular boulders and clay is a sign of ice, not of a river.

2. Moraines: ridges and sheets of till

• Moraine: a ridge or sheet of till left by a glacier.

Type	Where it forms	What it tells us
Terminal moraine	Across the valley at the toe (snout) of the glacier	The farthest point the glacier reached
Lateral moraine	Along the valley sides, parallel to the glacier	It may join the terminal moraine to make a horseshoe-shaped ridge
Medial moraine	Down the middle of a valley	Two glaciers met and their lateral moraines joined. It is less perfectly formed than a lateral moraine
Ground moraine	An uneven sheet of till over the valley floor	The glacier retreated fast and dropped its load unevenly

• How a terminal moraine forms:
• At the snout, the ice melts about as fast as it moves forward.
• Debris carried by the ice keeps getting dumped at the same line.

• Over time a ridge builds up there and marks the glacier's farthest advance.

• Recessional moraines: when a glacier retreats in steps, each pause leaves another ridge behind the terminal moraine. The Lake Michigan region (USA) has three regional recessional moraines left by the Lake Michigan ice lobe in the late Wisconsinan glaciation (the last glacial advance in North America) [9].

• Ice-thrust moraines: on Cape Cod (Massachusetts, USA) the Buzzards Bay moraine and the Sandwich moraine formed when an advancing ice front pushed over sediments it had laid down earlier [7].

3. Moraine-dammed lakes and GLOFs

• Moraines as dams: a moraine ridge across a valley can hold back meltwater and form a lake. Such lakes are used for water supply and hydropower.
• How these lakes form [2]:
• Glacial lakes form when glacial ice or moraines hold back (impound) water.
• The moraine leaves a hollow in which meltwater collects.
• The lake normally drains slowly by seepage (water leaking through the loose moraine) in front of the retreating glacier.

• If the dam is watertight, water keeps building up until seepage or overflow limits the lake level.

• Why they are dangerous:

• Till is loose and unsorted, so the moraine dam can be weak and unstable.
• If it breaks, a huge volume of water rushes out at once [2].

• GLOF (Glacial Lake Outburst Flood: a sudden flash flood caused when a glacial lake bursts its dam) [2].

• Climate link: as Himalayan glaciers shrink, more moraine-dammed lakes are forming [2]. Moraine-dammed lakes are the most common type of glacial lake in the Himalaya [4].

• Case study: South Lhonak Lake, North Sikkim, 3–4 October 2023
• South Lhonak is a moraine-dammed glacial lake in the Teesta basin. The Teesta is a major tributary of the Brahmaputra [2].
• Before the event, CWC had found that the lake had a very high probability of a sudden outburst. It modelled a flood about 175 km downstream to villages such as Lachen, Chungthang, Dikchu, Singtam, Rangpo and to the Teesta hydropower projects [2].
• Satellite images show that about 105 hectares of lake area drained between 28 September and 4 October 2023. ISRO compared a Sentinel-1A image (28 September) with a RISAT-1A image (4 October) [3].

• The burst caused a flash flood down the Teesta valley (details in natural-hazards-disasters).

• Chamoli, Uttarakhand, February 2021: another Himalayan flash flood, given in NCERT Class 9 ("Think About It") as a warning about glacial hazards.

• Monitoring in India:
• CWC made an inventory (official list) of glacial lakes and water bodies larger than 10 ha in the Himalayan region through NRSC, Hyderabad, in June 2011 [2].
• Lakes larger than 50 ha are checked monthly during the monsoon [2].
• Monthly monitoring reports have been published since June 2016 [4].

4. Eskers

• Esker: a long, winding (sinuous) ridge of coarse gravel and boulders.
• How it forms:
• In summer, meltwater flows in tunnels under the ice (subglacial) or inside it (englacial).
• The stream lays coarse gravel and boulders on the floor of its tunnel.

• When the ice melts away, this tunnel fill is left standing as a ridge that winds like the old stream.

• Esker material is stratified, unlike till, because water laid it [6].

• Some eskers left by Pleistocene ice sheets can be traced for hundreds of kilometres [6]. (Pleistocene: the Ice Age period, from about 2.6 million to 11,700 years ago.)
• Where seen: Canada, Finland and Sweden (Scandinavia), and the North European Plain.

5. Outwash plains

• Outwash plain: a broad, gently sloping plain at the foot of glacial mountains or beyond the edge of an ice sheet.
• How it forms:
• Meltwater streams leave the glacier front loaded with debris.
• As they spread out, they slow down and drop their load as fans.

• Many fans join to form one plain of gravel, sand, silt and clay, with coarse material near the ice and fine material farther away.

• Another name is sandur (plural: sandar), from Iceland, where large active outwash plains exist.

• World example: most of the Cape Cod landscape (Massachusetts, USA) is outwash plain, made of sand and gravel laid by meltwater streams [7].
• Outwash plain vs river alluvial plain (asked in NCERT Class 11):

Point	Outwash plain	River alluvial plain
Source of water	Glacier meltwater	Rain and snowmelt in a river basin
Material	Gravel, sand, silt and clay. Coarser and less well sorted	Mostly fine silt and clay. Well sorted
Location	At the glacier front or ice-sheet edge	Along a river's middle and lower course
Surface	Often pitted with kettle holes	Levees, meanders, oxbow lakes

6. Drumlins

• Drumlin: a smooth, oval hill made mostly of till, shaped like an upturned spoon. It is up to about 1 km long and 30 m high.
• Shape and direction:
• The long axis is parallel to the direction of ice flow.
• Stoss end: the blunt, steep end that faces up-glacier, where the ice came from.
• Tail: the gentle, tapering end that points down-glacier.

• So drumlins show which way the ice moved.

• How it forms: a moving glacier meets an obstacle or a heap of debris. It moulds the till around it into a streamlined shape, like water shaping a sandbar.

• Drumlins usually occur in groups of tens or hundreds called drumlin fields [5].
• "Basket of eggs" topography: a drumlin field looks like eggs lying in a basket. Examples are Ireland (County Down, Clew Bay) and Scotland.

7. Beyond NCERT: kames, kettles and Ice Age landscapes

• Kame: a lone mound of bedded sand and gravel that forms when stratified meltwater deposits slump and collapse as the ice next to them melts [6].
• Kame terrace: a similar bench that forms between the side of a glacier and the valley wall [6].
• Kettle (kettle hole): a hollow left by a buried block of ice [6].
• A block of ice breaks off the retreating glacier.
• Till or outwash is laid around and over it. The buried ice can survive for hundreds of years [6].
• When it finally melts, the ground above sinks and leaves a hollow [6].
• If the hollow reaches the water table (the top of the groundwater layer), it fills to form a kettle lake [7].

• Example: Great Pond, Wellfleet (Cape Cod). The organic mud at its bottom is dated to about 12,000 years ago [7].

• Kame-and-kettle topography: mounds and hollows mixed together, seen on Cape Cod [7].

• Pleistocene ice-sheet landscapes:
• Laurentide ice sheet: an ice sheet about 1.6 km (one mile) thick that covered most of Canada and the northern USA in the last ice age [8].
• The climate warmed about 20,000 years ago and the ice sheet retreated. Meltwater filled the basins the ice had scooped out, forming the Great Lakes. They reached their present shapes about 3,000 years ago [8].
• The Great Lakes hold about 20% of the world's fresh surface water and about 90% of the USA's [8].
• The ice reached its farthest point near Nantucket and Martha's Vineyard about 23,000 years ago and had left southern New England completely by about 15,000 years ago [7].

• Similar ice sheets in Scandinavia and across the North European Plain left wide spreads of till, moraine belts, drumlin fields and eskers.

• Davis's cycle of erosion link: glacial deposits interrupt the normal river cycle. When the ice goes, rivers start a new cycle on a surface of moraines, outwash and kettle lakes. This is why such landscapes have poorly drained, lake-dotted ground.

Prelims Hooks

• Till is unsorted, with angular to sub-angular fragments, laid directly by ice. Outwash is roughly sorted, stratified and somewhat rounded, laid by meltwater streams.
• Terminal moraine: at the snout. It marks the glacier's farthest advance.
• Medial moraine: forms where two glaciers merge. It is the least perfectly formed moraine.
• Ground moraine: an uneven till sheet left by a rapidly retreating glacier.
• Esker: a sinuous gravel ridge laid by streams in tunnels under the ice. It is a glacio-fluvial landform, not a moraine.
• Drumlin: the stoss end (blunt, steep) faces up-glacier and the tail points down-glacier. The long axis is parallel to ice flow. Drumlin fields give "basket of eggs" topography in Ireland and Scotland.
• Kettle lake: forms where a buried ice block melts. Kame: a mound of bedded sand and gravel [6].
• South Lhonak Lake (North Sikkim, Teesta basin) had a GLOF on 3–4 October 2023. About 105 ha of the lake drained [3]. The Teesta is a tributary of the Brahmaputra [2].
• CWC's glacial lake inventory covers lakes over 10 ha and was made through NRSC in June 2011. Lakes over 50 ha are monitored monthly in the monsoon [2].
• Trap: outwash plains and eskers are both stratified. Moraines and drumlins are unstratified (made of till).

Mains Points

• GLOFs are growing as glaciers retreat (GS-III, disasters): warming makes Himalayan glaciers shrink, so more moraine-dammed lakes form behind weak dams of loose till [2]. A breach sends a flash flood down valleys that hold villages, roads and hydropower projects. On the Teesta in 2023, the Teesta III project at Chungthang lay in the path of the flood [2][3]. Policy responses include the CWC–NRSC inventory, monthly satellite monitoring [2][4], sensor-based river gauges, early-warning sirens, and lowering lake levels.
• Glacial deposits as a resource and a constraint (GS-I): outwash plains are water-bearing aquifers (underground layers that hold water) and sources of sand and gravel. Cape Cod's outwash and kettle ponds are one example [7]. Moraine-dammed lakes supply water and hydropower. However, unsorted till and moraines are unstable ground for roads and dams in the Himalaya.
• Reading past ice ages: terminal moraines show how far ice once reached. Drumlins and eskers show which way it flowed. Kettle-lake sediments can be dated [7]. Together they rebuild the history of Pleistocene ice sheets, such as the formation of the Great Lakes [8], and help us understand how ice behaves as the climate changes today.