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L11 erosion 2 a.7


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Glaciation lessons for Edexcel A level Geography (2016 spec)

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L11 erosion 2 a.7

  1. 1. 1 1 2 2 33 5 6 5 4 4 Starter: Quick Quiz Numbers 1-10 in your books, name the features 1-6.
  2. 2. 7 Arête
  3. 3. 8 U-Shaped valley/ glacial trough
  4. 4. 9 Hanging valley 10 Truncated spur Bonus question: what scale category would these landforms be in?
  5. 5. Lesson title: Erosional landforms 2A.7 Glacial erosion creates distinctive landforms and contributes to glaciated landscapes N.B. Start of Enquiry Question 3
  6. 6. Erosional processes: • Erosional processes: abrasion, plucking (aka quarrying) • Dilation and fracture and traction (caused by crushing) • Meltwater flow (caused by basal melting) • Weathering: freeze-thaw aka frost shattering • Mass movement: Solifluction, creep
  7. 7. Erosional landforms we’ve already covered: • Cirques/corries, arêtes, pyramidal peaks, glacial troughs (U- shaped valleys), truncated spurs, hanging valleys and ribbon lakes --- all macro/meso/ micro features? These are all created by the processes previously covered in cirque/ valley glacier locations (temperate glaciers)- weathering (freeze-thaw), plucking, abrasion. Next we need to look at landforms formed by ice sheet scouring: 1. Knock and lochan (macro-scale) 2. Roches moutonées (meso) 3. Crag and tail (meso)
  8. 8. Term Scale Appearance Description Erosional Landforms Cirque/ corrie Arête Pyramidal peak U shaped valley (trough) Truncated spur Hanging valley Ribbon lake Crag and tail Roches moutonées Knock and lochan Striations Chattermarks For scale specify if it is macro, meso or micro scale. Focus on these today using resources provided. Complete others as revision task for HWK
  9. 9. Ribbon lakes • A ribbon lake is a long and narrow, finger-shaped lake, usually found in a glacial trough. They are macro-scale formations • Its formation begins when a glacier moves over an area containing alternate bands of hard and soft bedrock. the rate of erosion at the base will vary largely due to the underlying bedrock. • If the glacier moves over a softer rock it will erode more easily than a harder one. This creates a ‘stepped’ long profile (not completely smooth), with hollows called rock basins. • On either side of the rock basin, the more resistant rock is eroded less and these outcrops of harder rock are known as rock bars, which act as dams between which rainwater may accumulate after the retreat of the ice age, filling up the rock basin and creating a ribbon lake. • Another way they can form is by terminal moraines blocking the meltwaters route downstream and acting as a dam thus forming a lake behind it. • Sometimes these deep glacial troughs fill with seawater as the sea level rises as a result of melting ice increasing sea levels. These are known as fjords (flooded glacial valleys)- common in Norway (aka sea lochs in Scotland).
  10. 10. Scouring (erosional process) & knock and lochan Scouring occurs when ice sheets and glaciers expand out beyond constrained mountain valleys and erode large areas of lower relief land. This type of erosion is typical of warm-based ice (temperate glaciers) which are moving relatively slowly and eroding the bedrock beneath (subglacial). The landforms are largely dependent on the bedrock (hard/soft) leading to different rates in erosion. In Scotland landscapes to experience scouring are known as knock and lochan (macro-scale) as the higher areas of resistant rock (knocks) are interspersed with small lakes (lochans). Definition of knock and lochan: a glacially-scoured lowland area which displays alternating roches moutonnées (cnoc: a small rock hill in Gaelic) and eroded hollows often containing small lakes (lochans). Fracturing often determines the locations of linear valleys and rock basins. Linton(1963) coined this term to describe the highly irregular topography on the Lewisian gneiss of the NW Highlands and Outer Hebrides.
  11. 11. Roches moutonées (meso-scale erosional landform) ‘Sheep-like rocks’! These form as a result of glaciers eroding an outcrop of hard rock. They have a clear stoss (upside) and lee (downside) side. On the stoss side abrasion smooths the rock surface. On the leeward side (at the end of the outcrop) the pressure levels suddenly drop and a gap between the glacier and the bed may be present. Subgalcial meltwater fills this gap, refreezes onto the glacier base and plucks the underside creating a rough side (contrasting the smoothness of the top of the rock) Striations are common as subglacial material scratches the rock as the glacier passes over the top of it. e.g. Cairngorms, Scotland (300m long and 30m in height) Stoss Lee
  12. 12. Crag and tail (meso-scale landforms) • The most famous crag and tail is the site of Edinburgh Castle. The castle sits on an igneous intrusion which is incredibly resistant to erosion. • This hard rock provides a ‘pressure shadow’ on its lee side resulting in the soft rock behind it being eroded less than that around it. • Crag and tail landforms are normally larger than roche moutonées. • These can also be classed as depositional as sediment is sometimes deposited in the shadow zone creating a tail of till. glacial_erosion/revision/5/
  13. 13. Chattermarks Irregular chips and fractures in the rock . Chatter marks are commonly 1–5 centimetres (1/2–2 inches) but may be submicroscopic or as much as 50 cm in length. They occur mainly on hard, brittle rocks such as granite and are formed under a glacier by the pressure and impact of boulders moved along by irregular rolling or sliding. The resulting pattern of impacts has been likened to the “chatter” of a carpenter’s chisel slipping along the surface of a piece of wood. Chatter marks are commonly arranged in nested series, with the orientation of the fractures at right angles to the direction of glacial movement. Crescentic gouges are similar though more regular in appearance and are formed by the juddering effect of moving ice. The crescents are usually concave up-glacier (this image therefore indicates the glacier moved from top to bottom). Crescentic gouges on bedrock of gabbro, Loch Coruisk, Isle of Skye, Scotland. Photo M. J. Hambrey.
  14. 14. Striations Linear fine scratches formed by the abrasive effect of debris-rich ice sliding over bedrock. Intersecting sets of striae show differences in flow direction. Exceptionally well-preserved striated pavement of Permo- Carboniferous age (c. 290 m.y. old) near Douglas in the Great Karoo region of South Africa. Photo M. J. Hambrey.
  15. 15. Quick Quiz- erosional landforms 1. Give an example of knock and lochan. 2. Name three macro scale landforms. 3. What processes create roche moutonées? 4. What process creates chattermarks? 5. Give an example of an arête 6. Give an example of a pyramid peak. 7. How are striations formed? 8. What are the two main causes of ribbon lakes? 9. Give an example of a ribbon lake. 10. Draw a corrie.