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Glacial Deposition A2
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Glacial Deposition A2

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  • 1. Material deposited by glaciers is collectively termed GLACIAL DRIFT – It can be sub-categorised into: DIRECT DEPOSITION TILL - Glacial deposits, unstratified and unsorted material (boulder clay) – large fragments of rock of all sizes and shapes randomly mixed together. Can be divided into: 1. lodgement till – deposited at base of glacier due to basal melting – material is smeared onto the underlying rock due to the pressure of the ice as it continues to move 2. ablation till – material deposited as the ice melts away (coarser than lodgement till 2. INDIRECT DEPOSITION a. FLUVIOGLACIAL – picks up load once carried by ice (often then carried beyond limits of glacier – sorted and stratified by the action of water b. ICE – CONTACT STRATIFIED DRIFT – partly sorted by water – roughly stratified – deposited in vicinity of melting ice. GLACIAL DEPOSITION
  • 2. Features of Glacial Deposition: Till Deposits
    • Moraines
    • Drumlins
    • Till Plains
    • Erratics
  • 3. A DRUMLIN
  • 4. DRUMLINS
    • Characteristics:
    • Smooth elongated mounds of till – long axis parallel to the direction of the ice movement (mounds of glacial debris streamlined into elongated hills)
    • Where found in clusters – called “ drumlin swarm” – classic ‘basket of eggs topography’
    • Range in size from small mounds (2m high / 10m long) to huge hills – can be over a km long and 100m in height.
    • shape of drumlin measured using elongation ratio =
    • Length of drumlin
    • max width
    • Usually between 25:1 and 4:1 – greater elongation suggests more powerful ice flow.
  • 5. A Drumlin Field – also known as a Drumlin Swarm – typical “Basket of Eggs” topography
  • 6. Western Alberta, Canade
    • Theories of formation:
    • There are several theories for the formation of drumlins – and it is believed that different drumlins probably form in different ways (our understanding is incomplete. The formation is likely to include one or more of the following:
    • Formed as ice become overloaded – resulting in lodgement of subglacial debris as it melted out of the basal ice layers and further ice movement streamlines and moulds the material;
    • As glacier re-advances, previously deposited material is re-shaped
    • Accumulation of material around a bedrock obstruction (rock-cored drumlins);
    • Thinning of ice as is spreads out over lowlands resulting in reduction in competence of glacier – debris deposited and streamlined by continuing forward movement;
    • some believe catastophic floods beneath ice sheets ripple ground moraines into mounds / hollows.
    Examples of Drumlins : Hellifield, Ribblesdale – North Yorkshire – Swarms 40m high, 50-500m long – may be the result of ice sheets coming down from the Lake District, overloaded – change of gradient on the lowlands may have caused the ice to deposit the drumlins here. Other examples: New York State (USA) – largest drumlin field – 10,000 drumlins! Eden Valley - Cumbria
  • 7. Stoss End Lee Slope Direction of Ice Flow
  • 8. Lateral Moraine
    • Formed from debris frost shattered from the valley slopes – material falls onto surface of glacier and is carried along its edges.
    • - when melting takes place, an embankment of material is left against the valley sides.
    MORAINES
  • 9. Medial Moraine
    • Formed where two glaciers meet – resulting in the mergence of two lateral moraines.
    • material is often supraglacial and only about 1m of coarse debris
    • therefore, they rarely give signficant landforms in post-glacial periods.
  • 10. Terminal Moraine
    • This marks the max. extent of the glacier, forming at the snout.
    • - resembles large mound of debris (typical arc shaped) – shape dependent on:
    • 1. amount of material carried
    • 2. rate of ice movement
    • 3. rate of ablation
    • Few glaciers today have terminal moraine in contact with ice – due to rapid glacial retreat.
    • - terminal moraine, marks the boundary between glacial / proglacial areas (unsorted / angular) material.
  • 11. Recessional Moraine
    • Deposited by glacier as it retreats (if glacier stays still long enough for mound to build up a mount of material)
    • longer the pause – greater size of mound;
    • parallel to terminal moraine (90o to direction of movement)
    • - if there are several periods of retreat, there may be small sub-parallel ridges.
  • 12. Push Moraine
    • If the glacier readvances (e.g. due to climate deterioration) – previously deposited material may be shunted back up.
    • - this can be recognised by individual stones which are pushed upwards from their original horizontal position.
  • 13. ERRATICS
    • CHARACTERISTICS:
    • These are pieces of rock that geologically are out of place
    • They can vary from small pebbles to huge boulders.
    • FORMATION:
    • These rocks were initially supra-glacial debris (either from plucking or from rockfall from weathered slopes above the glacier.
    • They have then been transported and deposited into an area of differing rock type (hence the geological difference)
    • EXAMPLES:
    • Norber Erratics (Yorkshire Dales) – see photo – Blocks of Silurian shale – deposited on carboniferous limestone – erratics have protected the underlying rock from carbonation weathering, resulting in the erratics being perched on pedestal’s of the underlying rock.
    • Bluish Granite blocks unique to the island of Ailsa Craig (Ayrshire) found on the SW
    • Lancashire Plain – shows the direction of ice flow and a journey of at least 240km;
    • Chalk Rafts – transported from the North Sea bed to West Runton
  • 14. TILL PLAINS
    • Formed when large masses of unstratified drift, deposited at the end of an advance, smother the surface. It is an unsorted mixture of rocks, clays and sands.
    • How transported?
    • Largely transported as supraglacial debris – later deposited to form moraine – either during periods of active ice movement or when glacier was in retreat .
    • Description?
    • Individual stones are sub-angular – not rounded live river material and don’t posses the sharp edges of rocks recently broken up by frost-shattering;
    • Composition?
    • Composition of till reflects the character of the rocks over which it has passed – e.g. East Anglia – covered by chalky till – as the ice passed over a chalk escarpment.
  • 15.
    • Till Fabric Analysis
    • What is it?
    • This is a fieldwork technique used to determine the direction and source of glacial deposits
    • Involves measuring the orientation and dip of the long axes of a sample of 50-100 stones within the till – this is then plotted on a rose diagram
    • Stones and pebbles carried by glacier – tend to become aligned with their long axes parallel to the direction of ice flow – offers least resistance to the ice.
    • However – slumping; freeze-thaw and later ice-advances may disturb sediments in which case this type of analysis will not enable the reconstruction of the original direction of ice movement.
    Reference: All photographs used for educational use only – sourced from google images.