Geog cold environments 1

Ice Age - An extended period of time where the Earth experienced a cycle of very cold phases (known as glacials) interspersed with warmer phases (known as interglacials). The most recent ice age is called the Pleistocene Ice Age, began 2 million years ago.

Glacial - A very cold phase where global temps dropped by approx. 5 degrees resulting in ice covering 30% of the Earth's surface. The most recent advance was the Devensian.

Interglacial - A warm phase where temperatures increased to present levels or above which only lasts tens of thousands of years. The most recent retreat was the Ipswichian.

Present distribution of glacial ice and tundra

• 10% of Earth's surface is covered with ice.
• Ice sheets found in Antarctica & Greenland. - they lie at high altitudes.
• Found in North of the Arctic circle & South of the Antarctic circle.
• Tundra is found in the Northern hemisphere.
• Tundra countries: Alaska, Greenland, Canada, Iceland, Russia.

Plucking - A process of glacial erosion by which a glacier freezed around a rock on a valley, and subsequent movement of the ice causes the rock to be pulled away with it. E.G. Roche Moutonnee (has a smooth side & rough - the rough side has been plucked as glacier travelled over it)

Calving - The process of ablation by which small masses of ice break from an ice sheet or glacial. This can produce icebergs if the edge of the ice sheet extends into the sea. 

Sublimation - Involves a direct change of state from solid to a gas without becoming a liquid.

Ablation - A collective loss of water from an ice sheet or glacier. This loss can take a variety of forms. It is more dominant in the lower parts of a glacier and in summer (due to warmer temps) low altitudes.

Accumulation - The net gain in an ice mass. This gain can take place in a number of ways: precipitation, refreezing of meltwater; avalanches etc. It is more dominant in the upper part of the glacier and in winter (due to lower temps) and where high altitudes.

Glacial budget - The balance of ablation & accumulation in a glacier over 1 year. 

Formation of glacial ice

• Individual snowflakes have an open, feathery appearance. They are soft, light and fluffy due to the fact that they are 90% air. (Low density)
• When snow is piled up the weight causes air to be squeezed out. The increase in pressure also causes pressure melting. Tiny amounts of meltwater forms betwen snow crystals which then refreeze. Snow that has survived one year of freezing and melting is called firn or neve. (Medium density)
• Over time (20-40 years) the firn forms a solid mass of ice containing only around 20% air. (High density) 

Ice movement

Basal sliding. - WARM BASED GLACIERS

• Meltwater trickles through crevasses & moulins to reach the base of the glacier.
• Pressure from overlying weight of the ice causes the pressure melting point to lower so ice melts at temps lower than 0 degrees.
• The meltwater acts as a lubricant and therefore the glacier slides due to less friction. 

Extending & compressing flow.

• Ice moves depending on the shape of the valley floor.
• Extending flow - Ice fractures into thin layers which then slip downwards (steep part of glacier) - cracks form on glacier. 
• Compressing flow - Each layer slips forwards & fractures. (gentle part of glacier) - ice bunches up & thickens

Ice movement

Rotational movement.

• Occurs in a corrie, when the ice starts to move it follows the hollow shape.
• It rotates around an 'imaginary pivot point' - further deepens hollow.

Regelation creep

• Forms a roche moutannee. One side will have abrasion occuring - smooth surface (stoss side), the other side will have plucking occuring, this side will be jaggered (lee side).
• Base of the ice meets a rock outcrop on the valley flow. 

Internal flow - COLD BASED GLACIERS

• Overlying weight of the ice causes ice crystals to flatten & become more dense. this then makes them slide over each other in layers. (aka intergranular slip)
• Top layer = rigid layer. moves more due to less friction Lower layer is like plasticine & flows like molten plastic. (Flows least due to friction with bedrock)
• The weight & pressure causes the glacier to deform inside. 

Ice movement. (Substrate deformation) - When meltwater is present some of it will seep into the sediment below the glacier, if it becomes saturated it will act as a lubricant & the ice will slide.

Cold (polar) glaciers - Where the base temperature is well below 0 degrees and so remains frozen to the bedrock. Found in polar regions. e.g. Greenland, Antarctica. 

Warm (temperate) glaciers - The base temperature is around 0 degrees so it's not frozen to the bedrock. Found in temperate climates where there are obvious seasonal differences in temperatures. E.g. The Alps (Europe) 

5 different types of ice masses (smallest -> biggest)

• Niche glaciers - Very small & occupy hollows and gulleys on North facing slopes in the Northern hemisphere.
• Cirque (corrie) glaciers - Larger than niche glaciers. Small masses of ice occuping arm chair shaped hollows in mountains. Often overspill from their hollows to feed valley glaciers.
• Valley glaciers - Larger masses of ice which moves down from an ice field or a cirque basin source. Usually follow former river courses and are bounded by steep sides.
• Piedmont glaciers - Formed when valley glaciers extend onto lowland areas. This then spreads out and merges.
• Ice sheets and caps - Huge areas of ice which spread outwards from central domes. The whole landscape would be covered in ice (unless there's nunataks) - Ice sheets BIGGER than ice caps.

Glacial processes

Glacial EROSION 

• Glacial abrasion - Eroding of the valley floors and sides by the scraping & scouring of rock fragments as they are dragged along by the glacier as it moves.
• Plucking - Where glacial ice forms around fractured rock at the base or sides of the glacier. As the ice attatched to both the glacier and rock fragments, when the glacier moves it causes the rocks to be pulled away with it. 
• Freeze thaw weathering - A process of mechanical weathering in rocks which contain joints or cracks where the temps fluctutate around 0 degrees. When it's warm water will enter joints & then freeze when temps fall below 0 degrees. As the water freezes it expands by 9% which exerts pressure in the joint causing it to widen. This is a repeated processes of melting & freezing. Can lead to breaking of the rock.

Glacial processes

Glacial TRANSPORT 

• Supraglacial debris - Debris that is carried on the surface of the glacier. Normalled derived from scree & rock falls.
• Englacial debris - Debris that is carried within the glacier (embedded in the ice.)
• Subglacial debris - Debris which lies at the base of, or beneat a glacier or ice sheet.

Glacial DEPOSITION

• Ablation Till - Material that is left by the glacier as it retreats. It is the scattering of angular, unsorted debris which has 'melted out' of the ice as the snout retreats up the valley. (If the snout remained stationary then the would build up to form a moraine)
• Lodgement Till - A form of ground moraine which is deposited under a glacier/ice sheet. It can result in a drumlin forming.
• Erratics - A rock which has been transported by a glacier or an ice sheet and deposited in an area of different geology to that of its source. An erratic can be an indication of the direct of the ice sheet. E.g. Norber Moor erratics, North Yorkshire.

Arete - When 2 neighbouring corries run back to back. The glaciers erodes the sides of the valleys, sharpening the mountain ridge between them. E.g. Cwm Idwal, Snowdonia. 1km long & 250m high. KNIFE EDGED RIDGE.

Pyramidal peak - A pointed mountain peak with 3 or more sides. 3 or more corries will form back to back creating a point. E.g. Mount Snowdon. 1085m high.

Corrie - Snow collects in a hollow and turns to ice (N facing slope so snow can turn to ice due to no sun). Abrasion, plucking & basal sliding deep the hollow into an arm chair shaped hollow. E.g. Cwm Idwal, Snowdonia. 250m backwall & 1km diameter.

Roche moutonnee - The resistant mass of bedrock has a smooth stoss side due to abrasion and a steep, rough lee side due to plucking from the glacier as it travelled over it. E.g. Nant Ffrancon, Snowdonia. 12m long, 4m wide & 2m high.

Ribbon lake - Resistant rock erodes leaving long, thin dams or dips. After the glacier retreats lakes form. E.g. Llyn Ogwen, Nant Ffrancon. 

Tarn/corrie lake - Form in the corrie after the glacier has retreated.

Hanging valley - Waterfalls into the main glacial trough can be a result of hanging valleys. Tributary glaciers don't erode the valley floor as much as the glacier. When the glacier melts, leaving the valley hanging higher than the valley floor. E.g. Llyn Clyd, Nant Ffrancon. 

Fjord - Deep, long dips that form when the eroded valley floods due to rising sea levels.

Truncated spurs - Spurs (ridges of land) stick out and get chopped off (causing them to be truncated) due to the movement of the glacier. 

Glacial trough - The steep sided valleys with flat bottoms are formed when the glacier erodes the valley sides and wall deepening and widening them. E.g. Nant Ffrancon, Snowdonia. 5km long & 0.5km wide.

Corrie

Definition: A corrie is an arm chair shapped hollow in a mountain side with a steep backwall & rock lip.

Size: 1km diameter & 250m backwall.

Example: Cwm Idwal.

• Corries form when snow begins to accumulate in slight hollows on a North facing slope (no sunlight). Gradually the snow will begin to layer up & pressure will be put on it, turning it into glacial ice.
• FTW - Water gets into cracks and freezes which fractures the rock & then melts & refreezes eventually breaking the rock off, this weakens the back wall.
• Abrasion - Rock is grinded away making the corrie deeper. 
• Plucking - Ice freezes onto rock & then rips it off, this makes the backwall steeper.
• Cracks & joints are widened due to pressure release & this exaggerates the shape of the corrie. Mletwater flows down the cracks into base of glacier (basal sliding)
• Rotational movement - Ice moves in a rotational manner around an imaginary 'pivot point' - more erosion which deepens corrie & some debris deposited at the edge of the corrie which creates a rock lip.
• Floor of corrie uneven due to extending&compressing flow. Gradient steeper = ice thins. Gradient less = thickens.

Arete

Size: 1km long, 250m height

Example: Y Gribin, East Flank of Cwm Idwal.

Location: Between 2 corries, scree slopes at foot.

Shape: Knife edged ridge.

• 2 corries erode backwards into the same mountain & form a narrow knife edged ridge with steep sides found in upland glaciated regions due to the glacier eroding the sides of the valley.
• Result from the formation of corries & the arete represents the area remaining between 2 of these after they have been enlarged through glacial erosion & FTW.

Backwall which causes corriee to erode backwards.

Meltwater down the Bergschrund crevasse weakens the backwall through FTW & loosens material more easily which is eroded by abrasion. Then FTW results in fracturing of the rock in the backwall enabling even more weathering & erosion to take place. 

Pyramidal peak

Size: 1085m

Example: Mount Snowdon, The Matterham in Switzerland

Location: Mountain peak, surrounded by 3 corries

Features: Steep sides & several aretes radiate from central peak.

Shape: Isolated peak.

• 3 or more neighbouring corries develop, back to back on the side of a mountain leaving a very sharp mountain peak with steep sides and aretes radiating from the central peak.

Process with formation

• FTW - corries are enlarged as the backwall is broken down by FTW when ice gets into cracks in rocks & breaks off.
• Plucking- Rocks nearly break off from FTW so when ice moves past it, the rocks ripped off as ice attaches to rock & breaks it off. 
• Pressure release (dilation)- Less pressure on rock, it's weak so FTW&abrasion more @ work

Rochee Moutannee - Nant Ffrancon, Snowdonia, North Wales.

4m width, 2m high & 12m length

Formation

• Small bare mass of more resistant rock, rock outcrop is shaped by glacial erosion, with a smooth, gentle sloping side (stoss slope) & steep, rough, irregular (lee side)
• smooth, rounded upvalley or stoss slope is facing the direction of ice flow & a steep jaggered lee side created by plucking.
• As the glacier flows over the stoss slope, angular material can scratch the rock creating striations (subglacial & englacial material causes abrasion).
• High pressure on the stoss slope causes ice at the bottom of the glacier to melt (as it reaches it's PMP)
• as glacier moves further up the stoss slope there is a reduction in pressure, causing the water to refreeze on the lee side.
• this refreezing of water leads to plucking - causing a jaggered lee side.

Glacial trough - Nant Ffrancon

0.5km wide, 5km long

• When a glacier calves through a valley through abrasion, plucking & FTW it creates a U-shaped valley, with steep sides & flat valley floor.
• The previous V-shaped valley was widened, deepened & steepended by the glacier and any interlocking spurs were truncated, leaving truncated spurs as a major feature of the valley.
• Extending & compressing flow as well as non resistant & resistant rock can create a stepped valley long profile. This is because higher gradients = ice will move faster, extend & thin so less erosion takes place. Gentler gradients = ice will slow and compress thus thicken having more erosive power.
• difference in erosive power (Differential erosion) will further exaggerate differences in gradient, creating a stepped valley long profile.
• this stepped valley long profile can also be created by areas of resistant rock and non resistant rock

Hanging Valley.

• can also feature in a glacial valley, where a tributary glacier doesn't erode as much as the main glacier it feeds.
• postglacially as the ice melts, the tributary glacier's valley is left hanging above the main valley.
• Postglacially, a river can appear in the glacial trough, that is clearly too small to have eroded the valley itself, and so therefore is called a misfit river. 
• Also the continued action of FTW may create scree slopes that decrease the gradient/steepness of valley sides.

Glacial deposition 

Moraine - A moraine is any till (material) that has been deposited directly by the glacier. 

Till fabric analysis: 

• Angular material (due to no attrition)
• Unsorted material: this means there are large fragments (boulders) and small material (clay) all mixed together.
• No orientation: The material is jumbled up.
• Not stratified: Shows no signs of layering (push moraines are an exception & drumlins too)

Types of moraine

• Lateral: Deposited where the sides of the glacier were. e.g. sides of Cwm Idwal.
• Medial: Deposited in the centre of the valley where 2 glaciers join together.
• Terminal: Builds up at the end of the glacier & is deposited as semicircular hills of till.
• Recessional: Forms behind the terminal moraine.
• Push: Found in the centre of a glacier resulting from 2 lateral moraines merging.

Drumlins. Ransa Hills, North Yorkshire. 25m high, 50m wide & 150m long

Stoss end: wide & tall.  Lee slope: long & narrow.

Drumlins are smooth elongated mounds of till with their long axis parallel to the movement of the ice. They are always longer than they are wide and are usually found in swarms. (basket of eggs)

Formation:

• Ice became overloaded with debris reducing the capacity of the glacier. 
• This reduced the competence of the glacier, may be due to melting or changes in flow (e.g. extending or compressing flow) 
• after the till have been deposited it may be moulded or streamlined by later ice movements.

Other theories

• There may be due to deposition around a central core, although not all drumlins have this.
• meltwater is also thought to contribute although they are often made up of unsorted till.

Glacier is overloaded with subglacial debris. During glacial retreat, the glacier at its snout is thinning, resulting in a reduced capacity & competence. Deposition of subglacial debris occurs at the base of the glacier. Continued movement of the ice over the deposited till means that it's streamlined.

Erratics

Norber Moor erratics, North Yorkshire

Erratics are rocks that have been carried a long way by glaciers and deposited in a place with different geology. 

• Sometimes the sediments in till will contain large boulders. If these boulders have travelled a long way from their place of origin they are known as erratics.
• They're used as evidence for ice movement. Rocks found in one area have a different composition/different rock type showing that they clearly do not originate from that area.

Glacial deposition

• As the ice melts the material which has been carried by the glacier will be left behind. The material deposited by the glacier in this way is known as glacial till. 

Till - Is an unsorted mixture of clay, sand, gravel & boulders which are deposited directly from the moving ice. Till deposits can vary considerably in sediment structure, orientation and size - reflecting the different processes involved in their formation.

Ablation till - Material which is left behind by the glacier as it retreats. It's a scattering of angular, unsorted debris which has melted out of the ice as the snout retreats up the valley.

Lodgement till (ground moraine) - Subglacial material deposited by the actively moving glacier. Form of moraine which is deposited under the glacier. The rock particles are laid down under the pressure of the slowly moving ice when the base melts. The till stones may display some orientation with the long axis in the direction of ice movement.

Describing glacial till deposited directly by the ice:

• Unsorted, Not stratified, Not graded & Angular (not rounded) 

Moraine are ridges of debris that develop along the margins of glaciers in response to the dumping of sediment that has been transported to the front and edge. 

End moraine - Moraine found at the snout of the glacier. (Umbrella term for 3 main types: recessional, terminal & push moraine)

Lateral moraines - Along the valley sides. Formed from debris drived from frost shattering of valley sides and carried along the edges of the glacier. 

Medial moraines - Found in the centre of a valley amd results from the merging of two lateral moraines where 2 glaciers have joined. Orientation of the till is in the direction of the glacial advance. 

Moraine is made up of unconsolidated material, till (any material deposited directly by the glacier) 

Lateral moraine. 25m high & 1km long. Cwm Idwal, Snowdonia

Ridges of glacial till found along sides of the glacier. Material is elongated in the direction of ice movement along the valley floor. Material is also unsorted, angular, unstratified & has no orientation.

• Forms along the side of the glacier. Material from the valley walls is broken up by frost shattering & it will fall onto the ice surface as supraglacial debris. 
• It's then carried along the sides of the glacier. Some of the material may even be brought from further up the valley by plucking (ripped off the valley sides as glacier freezes to the bedrock & then moves pulling loose rocks with it) and abrasion (grinding action of the bedload on the valley floor & sides)
• when the ice melts & retreats, it forms a ridge of material along the valley sides.

Moraine 

End moraine - often used to describe the moraine at the snout of the glacier. There's 3:

Terminal moraine - 

• Deposited which marks the furthest advance of a glacier. Can be up to 50m high & 1/2km wide. They are crescent shaped ridges (due to the shape of the glaciers snout). They form high mounds with a steeper up valley side on a gentler down valley side.
• They form across the valley floor on low land areas. The till is unsorted, angular, unstratified & it has no orientation.
• They are often dissected by meltwater streams. Lakes can form in front of the glaciers, when the flow of the melt water streams get dammed by the terminal moraine.

Recessional moraine - 

• Form behind and parallel to the terminal moraine, marking the interruptions in glacial retreat when the glacier has remained stationary for long enough for a ridge of till to develop.

Push moraine - Previously deposited end moraine shunted forwards by an advancing glacier. Material is deposited at the snout of a glacier when it remains stationary for long enough for a ridge of till to form. If the ice then advances the material is pushed forwards in the direction of ice movement, till is orientated in the direction of the ice advance.

Formation of moraines 

• End moraines form at the snout of a glacier when it has remained stationary for a long enough period of time for deposition of subglacial, supraglacial and englacial debris to build up and form a ridge - due to ablation and accumulating being equal.
• The debris comes from abrasion (grinding action of bedload carried by glacier on valley floor and sides) and plucking (when the glacier freezes to the valley sides and rips off loose blocks as it moves)
• The ice then acts as a conveyor belt as it is always moving subglacial, englacial and supraglacial debris to its snout - even when the glacier is stationary. Much of the material is carried within the ice and then deposited at its snout if it melts.
• Subglacial and valley floor deposits are thrust upwards to form a steep up valley slope and a more gentle slope down valley.

• Drumlins are smooth, round, elongated hills which are elongated in the direction of ice movement. They have a steep stoss face (up) & a gentle lee side (down valley). The glacial till is angular, unsorted, unstratified & with possible orientation with the long axis in the direction of ice movement.
• they are usually found in lowland areas on valley floors where ice has emerged from highlands. They will usually occur in large numbers known as swarms "basket of eggs"
• They are made up of fine clays to boulders, often poorly sorted and angular and unstratified. There may be some orientation of till at the surface due to stream lining caused by the glacier moving over them.
• E.G. Ransa Hills, North Yorkshire.

Explanation 

• Result of ice being overloaded with debris. This reduces its capacity and ability to carry and deposition occurs at the base of the ice. Once deposited the material is streamlined further by the ice advance. 
• The drumlin isn't simply eroded by the ice because the ice is carrying too much debris and has low capacity and competence - the ice at this point does not have the erosional power or velocity to erode the drumlin. 

Outwash plains - Sandurs e.g. Skogasandur, Iceland

Sandurs - Located in front of the glacier snout. They are large plains of deposited till/sediment. The till is rounded, well sorted, graded & stratified. 7km from front to sea. 21km across. 50m deep.

• meltwater streams entrain, erode & transport material
• meltwater streams carry a large load capacity & competence
• The streams slow down when they reach the snout and deposit their rounded load to form the outwash plain.
• material is deposited following hjulstroms curve so that the finest material is deposited on the outwash plain.
• till is well sorted and graded as well as vertical layering due to seasonal variation.
• Very small slope angle, less than 4 degrees.
• braided streams. 

Explanation - 1) Increased friction due to reduction in velocity of meltwater streams at the snout. As velocity drops with distance from snout, so will capacity & competence so larger particles will be deposited 1st. 

2)in summer there's more meltwater so the stream has higher capacity and competence carrying courser (rough) particles then in winter its the opposite resuling in vertical layering.

3)meltwater streams braid due to high seasonal variability. At high velocity, lots of material is transported, when there's a low discharge, streams have a high load in relation to their capacity so the material is deposited within the channel.

Eskers. - Long, sinuous (winding), ridges (mounds) composed of sand & gravels. Represent channel deposits of subglacial streams & englacial streams.

120m length, 10m wide & 5m high. Wensleydale, North Yorkshire. Right angle to ice front of parallel to ice movement. Can combine with mounds of material (kames) to form beaded eskers. Till is rounded, sorted, graded & stratified (vertical layering) 

Formation

• Till is carried in subglacial & englacial meltwater streams.
•  Meltwater streams energy is reduced so deposition in the tunnel occurs (in winter)
• Deposition occurs on the stream bed within confined ice walls.
• Running water sorts the sediment and the deposits are also stratified, ( in summer, gravel due to greater discharge. In winter sand due to lower discharge) 
• Yearly cycle creates graded bedding. 
• When the glacier melts, the tunnel deposits sand and gravel which collapse at 30 degrees forming a long sinuous ridge along the valley floor. 

Kame HILLS. They're small mounds made of sand & gravel formed where a meltwater stream flows into crevasses. The supraglacial meltwater stream doesn't have an easy exit out of crevassel variability. (Summer = higher competence = large particles deposited. Winter = lower discharge, lower competence = small particles deposited). If the glacier melts, the deposits remain but they collapose & slump downwards.

Kame TERRACES. These are ridges of of sand & gravel that run along the sides of a valley between the valley walls & the glacier. They valley sides that are exposed heat up which melts some of the ice next to it creating a small tough and a meltwater stream at the side of the glacier. As the meltwater stream loses energy and competence it deposits it's load between the glacier & the neighbouring valley wall, the deposits are sorted and stratified, again due to seasonal variability. Kame terraces look similar to lateral moraines except a kame is made from difference material - it's always made from fluvial glacial deposits. s so it loses velocity & energy which causes deposition - the stream deposits it's load in the crevasse. The deposits are rounded due to attrition (rocks bash together & break off) and stratified due to seasonal variability.

Kame DELTAS. Small mounds of deposited material infront of a proglacial lake. Material that's carried by a meltwater stream gets deposited because the stream spreads out when leaving the glacier so friction is increased which reduces the streams competence. It then deposits the load which is sorted & stratified due to seasonal variability. Triangular shape & can be seen better if glacier melts.

Varves - Alternating layers of sediment deposited in a proglacial lake or kettle lake. 

• In Spring & Summer months the discharge of meltwater streams is much greater due to the warmer temps amongst alpine glaciers, this leads to increased competence & capacity.
• On entering a lake the velocity of the stream is reduced depositing its mainly coarse material, which forms a lighter band of sediment on the lake bed. 
• In Autumn & Winter the discharge of the meltwater streams is much lower due to the colder temps so the competence & capacity is much lower. On entering the lake the stream deposits its load of mainly finer material which forms a darker band of sediment on the bed of the lake. 

E.g. Preston Montford Fieldcentre

Proglacial lakes, overspill channels & drainage diversion.

Proglacial lake - A meltwater lake occuring on the edge of the ice front of a glacier or ice sheet. The ice acts as a natural dam and blocks the course of a river.

Preglacially the River Severn, Wales, flowed Northwards into the Dee Estuary & the lower Severn flowed from the Welsh boreders to the Bristol Channel. 

During glaciation, the river was unable to flow North deu to the ice front acting as a dam, this caused a proglacial lake to form; Lake Lapworth. The evidence of this is the presence of varves.

The level of water in the proglacial lake built up until it was able to breach the lowest point (the col) of the drainage basin. The Ironbridge gorge was formed when the high energy meltwater in Lake Lapworth drained through the Souther drainage basin to join the original Lower Severn. The water flowed into an overspill channel where there is rapid vertical erosion & cut through a solid rock area creating the gorge.

Post glaciations the River Severn flows from central Wales to the Bristol Channel. This is because the preglacial route North was blocked by glacial deposits. Also because a lower channel had been eroded so the water continued flowing South.

Proglacial lakes

As a glacier begins to retreat, there are large amounts of meltwater flowing to the snout. This meltwater can be dammed by terminal moraines so it builds up to form a proglacial lake in front of the glacier. Alternatively, river valleys that flow towards a glacier can be blocked by the ice, so it has nowhere to go & builds up to form a proglacial lake, also infront of the ice. 

• Small amounts of meltwater accumulates at the snout of the glacier, trapped between the ice & the rock or a terminal moraine. 
• As the glacier melts & retreats, the meltwater has nowhere else to go but into the small lake, which grows in size as more water is trapped. 
• There is still ice blocking the meltwater which has now accumulated to form a large proglacial lake.