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Glacial Deposition A2


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Glacial Deposition A2

  1. 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. 2. Features of Glacial Deposition: Till Deposits <ul><li>Moraines </li></ul><ul><li>Drumlins </li></ul><ul><li>Till Plains </li></ul><ul><li>Erratics </li></ul>
  3. 3. A DRUMLIN
  4. 4. DRUMLINS <ul><li>Characteristics: </li></ul><ul><li>Smooth elongated mounds of till – long axis parallel to the direction of the ice movement (mounds of glacial debris streamlined into elongated hills) </li></ul><ul><li>Where found in clusters – called “ drumlin swarm” – classic ‘basket of eggs topography’ </li></ul><ul><li>Range in size from small mounds (2m high / 10m long) to huge hills – can be over a km long and 100m in height. </li></ul><ul><li>shape of drumlin measured using elongation ratio = </li></ul><ul><li>Length of drumlin </li></ul><ul><li>max width </li></ul><ul><li>Usually between 25:1 and 4:1 – greater elongation suggests more powerful ice flow. </li></ul>
  5. 5. A Drumlin Field – also known as a Drumlin Swarm – typical “Basket of Eggs” topography
  6. 6. Western Alberta, Canade <ul><li>Theories of formation: </li></ul><ul><li>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: </li></ul><ul><li>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; </li></ul><ul><li>As glacier re-advances, previously deposited material is re-shaped </li></ul><ul><li>Accumulation of material around a bedrock obstruction (rock-cored drumlins); </li></ul><ul><li>Thinning of ice as is spreads out over lowlands resulting in reduction in competence of glacier – debris deposited and streamlined by continuing forward movement; </li></ul><ul><li>some believe catastophic floods beneath ice sheets ripple ground moraines into mounds / hollows. </li></ul>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. 7. Stoss End Lee Slope Direction of Ice Flow
  8. 8. Lateral Moraine <ul><li>Formed from debris frost shattered from the valley slopes – material falls onto surface of glacier and is carried along its edges. </li></ul><ul><li>- when melting takes place, an embankment of material is left against the valley sides. </li></ul>MORAINES
  9. 9. Medial Moraine <ul><li>Formed where two glaciers meet – resulting in the mergence of two lateral moraines. </li></ul><ul><li>material is often supraglacial and only about 1m of coarse debris </li></ul><ul><li>therefore, they rarely give signficant landforms in post-glacial periods. </li></ul>
  10. 10. Terminal Moraine <ul><li>This marks the max. extent of the glacier, forming at the snout. </li></ul><ul><li>- resembles large mound of debris (typical arc shaped) – shape dependent on: </li></ul><ul><li>1. amount of material carried </li></ul><ul><li>2. rate of ice movement </li></ul><ul><li>3. rate of ablation </li></ul><ul><li>Few glaciers today have terminal moraine in contact with ice – due to rapid glacial retreat. </li></ul><ul><li>- terminal moraine, marks the boundary between glacial / proglacial areas (unsorted / angular) material. </li></ul>
  11. 11. Recessional Moraine <ul><li>Deposited by glacier as it retreats (if glacier stays still long enough for mound to build up a mount of material) </li></ul><ul><li>longer the pause – greater size of mound; </li></ul><ul><li>parallel to terminal moraine (90o to direction of movement) </li></ul><ul><li>- if there are several periods of retreat, there may be small sub-parallel ridges. </li></ul>
  12. 12. Push Moraine <ul><li>If the glacier readvances (e.g. due to climate deterioration) – previously deposited material may be shunted back up. </li></ul><ul><li>- this can be recognised by individual stones which are pushed upwards from their original horizontal position. </li></ul>
  13. 13. ERRATICS <ul><li>CHARACTERISTICS: </li></ul><ul><li>These are pieces of rock that geologically are out of place </li></ul><ul><li>They can vary from small pebbles to huge boulders. </li></ul><ul><li>FORMATION: </li></ul><ul><li>These rocks were initially supra-glacial debris (either from plucking or from rockfall from weathered slopes above the glacier. </li></ul><ul><li>They have then been transported and deposited into an area of differing rock type (hence the geological difference) </li></ul><ul><li>EXAMPLES: </li></ul><ul><li>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. </li></ul><ul><li>Bluish Granite blocks unique to the island of Ailsa Craig (Ayrshire) found on the SW </li></ul><ul><li>Lancashire Plain – shows the direction of ice flow and a journey of at least 240km; </li></ul><ul><li>Chalk Rafts – transported from the North Sea bed to West Runton </li></ul>
  14. 14. TILL PLAINS <ul><li>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. </li></ul><ul><li>How transported? </li></ul><ul><li>Largely transported as supraglacial debris – later deposited to form moraine – either during periods of active ice movement or when glacier was in retreat . </li></ul><ul><li>Description? </li></ul><ul><li>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; </li></ul><ul><li>Composition? </li></ul><ul><li>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. </li></ul>
  15. 15. <ul><li>Till Fabric Analysis </li></ul><ul><li>What is it? </li></ul><ul><li>This is a fieldwork technique used to determine the direction and source of glacial deposits </li></ul><ul><li>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 </li></ul><ul><li>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. </li></ul><ul><li>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. </li></ul>Reference: All photographs used for educational use only – sourced from google images.