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Erosional Forms and Landscapes

Erosional Forms and Landscapes. chapter 9. Erosional Landscapes. Areal Scour vs. Selective Linear Erosion. Cirques. Cirque Form. Exponential Process Overdeepened Max work @ ELA Tarns. Cirque Orientation. Any orientation is possible Commonly to NE in Northern Hemisphere.

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Erosional Forms and Landscapes

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  1. Erosional Forms and Landscapes chapter 9

  2. Erosional Landscapes • Areal Scour vs. • Selective Linear Erosion

  3. Cirques

  4. Cirque Form • Exponential • Process • Overdeepened • Max work @ ELA • Tarns

  5. Cirque Orientation • Any orientation is possible • Commonly to NE in Northern Hemisphere

  6. Cirque Orientation • Any orientation is possible • Commonly to NE in Northern Hemisphere

  7. Why to NE? • Insolation + sensible heat transfer? • Effect of wind drifting? • ? test in southern hemisphere (New Zealand) ?

  8. Cirques Alex Marienthal March 13, 2009

  9. Form and Morphology

  10. Many Forms

  11. Form • Long Profile • Arc of circle? • Logarithmic! • y= k (1 – x) e-x (Haynes,1968) • k is constant • L/H ratio Haynes, 1968

  12. Backwall and Foot Slope

  13. Cirque Floor • Overdeepened • Reverse slope • f (dip) • Floors follow pattern • Torridonian Sandstone • North West Highlands, Scotland Haynes, 1968

  14. Process • Rotational Flow • Headwall • Back movement • Floor • Overdeepening • ELA • Max. erosion

  15. Backwall – processes?

  16. ELA • Cirque vs. valley glacier • Altitude

  17. References • Andrews, J.T., 1975, Glacial Systems, Duxbury Press, North Scituate, Mass. • Battle, W.R.B., 1960, Temperature observations in bergschrunds and their relationship to frost shattering, in Lewis, W.V. (ed.), Norwegian Cirque Glaciers, Royal Geographic Society Research Series 4, p. 5-10. • Benn, D.I., and Evans, D.J.A., 1998, Glaciers and Glaciation, Arnold Press, New York • Evans, I.S., 1969, The geomorphology and morphometry of glacial and nival areas, in Chorley, R.J. (ed.), Water, Earth, and Man, Methuen, London, p. 369-380. • Gardner, J.S., 1987, Evidence for headwall weathering zones, Boundary Glacier, Canadian Rocky Mountains, v. 33, p. 60-67. • Haynes, V.M., 1968, The influence of glacial erosion and rock structure on corries in Scotland, Geografiska Annaler. Series A, Physical Geography, v. 50, p. 221-234. • Hooke, R.L., 1991, Positive feedbacks associated with erosion of glacial cirques and overdeepenings, Geological Society of America Bulletin, v. 103, p. 1104-1108. • Sugden, D.E., and John, B.S., 1976, Glaciers and Landscape, Wiley, New York.

  18. Arêtes and Horns • Jointing and mass wasting (two cirques)

  19. Arêtes and Horns • Jointing and mass wasting (two cirques) • Coalescence of three or more cirques

  20. Washington Cascades • ridge and average elevations

  21. Glacial buzz-saw: do average cirque elevations  Cascade erosion?

  22. are there cirques on Cascade volcanoes? why or why not?

  23. Troughs • “U” shaped • Like a stream, but slower! • X-section area = f(Q)

  24. Troughs • “U” shaped • Like a stream, but slower! • X-section area = f(Q) • Elevation = f(Q) at common surface

  25. Trough Erosion • Scour @ bed • Mass wasting @ sides (transport)

  26. Trough Erosion • Scour @ bed • Mass wasting @ sides (transport)

  27. Trough Evolution • Real form

  28. Trough Evolution • Real form • Modeled form (Harbor, 1992, GSAB)

  29. Exponential Form • “b” = exponent • Form Ratio = valley width/ valley depth

  30. Trough Erosion • Erosion = f (effective pressure) • “Effective” N = f (water pressure)

  31. Trough Erosion • Erosion = f (effective pressure) • N = f (water pressure) • Erosion = f (velocity)

  32. Trough Erosion • Erosion = f (effective pressure) • N = f (water pressure) • Erosion = f (velocity) • Morphology = complex function!

  33. Trough Evolution • Modeled by Harbor (1992) • Results in “realistic” erosion • Sequence = less realistic!

  34. Trough Evolution • Modeled by Harbor (1992) • Results in “realistic” erosion • Sequence = less realistic!

  35. Model Evolution • “Equilibrium glacial trough” • “b” value between 2 and 2.5 • Form ratio unstable (mass wasting?)

  36. Paternoster Lakes • Local overdeepenings • Rel. erodibility? • Extension/ compression? • Some evidence of cyclicity

  37. Fiords • Definition: • Drowned glacial troughs • Appearance: • Steep walls rising from the sea

  38. Fiords

  39. Trough Lake = Fiord? • Two Medicine Lk • Fiordland (NZ)

  40. Thresholds and Strandflats • Overdeepen at confluences • Rise to threshold Strandflat Threshold Moraine? Moraine?

  41. Areal Scour (ice sheet) • Depends heavily upon basal processes = f(T) • Results in a suite of landforms • May show superimposed patterns

  42. Areal Scour (ice sheet)

  43. SLE(ice sheet) • Examples: Finger Lakes • Selective linear erosion • Edge of Allegheny Plateau • Possibly locally wet-based; feedback? • Fluvial?

  44. SLE(ice sheet) • Examples: Finger Lakes • Selective linear erosion • Edge of Allegheny Plateau • Possibly locally wet-based; feedback • Not the only such example!

  45. Breached Divides • New England “notches” • Ice advances through notch • Subglacial drainage?

  46. Puget Lobe

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