Glacial Diamicton: Till

General characteristics

• Heterogeneous: extremely poorly sorted containing both clay and large boulders; may exhibit a bimodal or multimodal distribution which commonly results from the crushing of rocks composed of mineral grains or from selective transport.
  • Composed of: lithic fragments, mineral grains, crushed mineral grains
• Massive: rarely exhibits internal laminae.
  • However, meltwater deposited laminae may be present.
  • May also exhibit a fissility if deformed by subglacial brittle-ductile deformation.
• Except for some deformation tills and glacitectonites, tills typically contain erratic material, that is, clasts having a composition that is different that the underlying bedrock. Deformation tills are produced by the deformational dissagregation of underlying lithology.
• Clasts shape varies depending on transport location within a glacier and distance traveled and lithology.
  • Most clasts transported in a subglacial environment are subangular to subrounded and asymmetrical.
  • Clasts transported entirely on the surface of the glacier will be angular.

• Lodgment Till (ice-transported debris deposited from the base of active ice)
  • Processes
    • lodgement of debris in traction (basal shear)
      • occurs when friction resistance between the sediment and the bed overcomes the tractive force of the glacier
      • obstacle related--may occur on the stoss or less-side of an obstacle depending on effective stress
      • factors affecting deposition:
        • bed roughness
        • high debris concentration in basal layers causing stagnation and detachment
        • high basal water content under a high hydrostatic pressure reduce frictional drag and inhibit till deposition
    • release by pressure melting
      • Pressure melting incrementally released sediment from the base of the glacier.
    • Subglacial flowage into cavities
      • saturated till is squeeze in to areas of low pressure (lee-side cavities, crevasses, etc.)
  • Characteristics of lodgement till
    • Typically compact (over consolidated)
    • Fissile or penetratively cleaved
    • Clasts are be striated, faceted, and subrounded (bullet-shaped)
    • Typically exhibits a fabric with a preferred orientation of clasts
    • Clasts are asymmetrical with abraded and plucked up- and down-ice ends.

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• Melt-out Till (deposited from stagnant ice)
  • Subglacial melt-out till
    • Subglacial melt out tills may be difficult to distinguish from lodgement till if melting was slow and original fabrics remained intact [Compare with Clayton and Moran p. 98)
  • supraglacial melt-out till
    • clasts are typically courser, angular, and randomly oriented.
  • Sublimation till: Sediment is released by the vaporization of water from cold arid glaciers. Includes both suglacial and supraglacial material.
  • till is loose and preserves the structure of the encasing ice
  • easily remobilized

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• Subglacial deformation of underlying sediment

• Glacitectonite: subglacially deformed rock or strata the retains some of its original structure.
• Deformation till: intense shearing of poorly consolidated or unlithified bed produces homogenized, cleaved, over consolidated till with a strong clast orientation
  

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• Resedimented till
  • Flow till (mass-wasted, debris flow)
  • Ice-rafted till
  • Water-lain till
  • marginal deformation till (deformed along the margin by bulldozing and thrusting)

Problems: A genetic classification of tills is often impossible without a detailed analysis of its fabric, lithology and facies associations.

Subglacially deposited

• Ground moraine: undulatory sheet covering the ground--smoothes out preexisting topographic irregularities (e.g. valley become filled)
• Streamlined hills or ridges parallel to flow
  • Till "drumlin" (not all drumlins are till but most have a till carapace):
  • Flutes: Small ridges extending down-ice from boulders and aligned parallel to ice flow (c.f. crag and tail)
    • Unlike crag and tail lineations flutes are depositional
    • Formed by the release or squeezing of till in lee-side cavities
    • Not often preserved
  • Crevasse-fill ridges: orientation varies according to the type of crevasse
• Subglacial ridges trasverse to flow
  • rogen moraines

Supraglacial and ice marginal features

• Hummocky topography: formed by the melting of debris rich stagnant ice.
  • Till mass-wastes into depressions.
  • Stagnant-ice relief is topographically inverted when ice melts.
  • Commonly associated with fluvial ice-contact and lake deposits lain down in the stagnant ice.
  • A type of hummocky topography may also be composed largely of glacifluvial sediments (kame and kettle topography).
  • Characteristic of large end moraines, especially in the Midwest
• Till rings: Generally associated with hummocky topography
• Till "moraines"
  • end moraines, lateral moraines, etc.