7.3 Glacial deposition and sedimentary structures

Glacial deposition shapes landscapes through complex processes. Ice sheets and glaciers leave behind a variety of sediments and landforms, from unsorted till to stratified outwash plains. These deposits provide valuable clues about past ice movements and environmental conditions.

Understanding glacial deposits is crucial for reconstructing Earth's climate history. By analyzing sediment composition, structures, and landforms, scientists can piece together the extent of ancient ice sheets, their flow patterns, and the dramatic changes they brought to the landscape.

Glacial Deposit Types

Till and Moraines

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  Alpine glacial till (Pleistocene; near Dana Fork, Yosemite… | Flickr View original

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  ESurf - Sediment supply from lateral moraines to a debris-covered glacier in the Himalaya View original

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  Alpine glacial till (Pleistocene; near Dana Fork, Yosemite… | Flickr View original

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  ESurf - Sediment supply from lateral moraines to a debris-covered glacier in the Himalaya View original

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Top images from around the web for Till and Moraines

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  Alpine glacial till (Pleistocene; near Dana Fork, Yosemite… | Flickr View original

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  ESurf - Sediment supply from lateral moraines to a debris-covered glacier in the Himalaya View original

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  Alpine glacial till (Pleistocene; near Dana Fork, Yosemite… | Flickr View original

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  ESurf - Sediment supply from lateral moraines to a debris-covered glacier in the Himalaya View original

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• Till consists of unsorted sediment mixture deposited directly by glacial ice ranging from clay-sized particles to large boulders
• Moraines form distinct landforms of accumulated glacial debris
  • Terminal moraines mark the farthest extent of a glacier
  • Lateral moraines develop along glacier sides
  • Medial moraines form where two glaciers merge
• Erratics appear as large boulders transported by glaciers and deposited far from their source often used as indicators of ice flow direction

Glaciofluvial and Glaciolacustrine Deposits

• Glaciofluvial sediments result from sorted and stratified deposits formed by meltwater streams
  • Outwash plains develop as broad, gently sloping surfaces of glacial sediment
  • Kames form as mounds of stratified drift deposited by meltwater
  • Eskers appear as long, sinuous ridges of stratified sand and gravel deposited by subglacial streams
• Glaciolacustrine deposits form in proglacial lakes consisting of fine-grained sediments
  • Often exhibit rhythmic layering called varves
  • Varves represent annual cycles of sedimentation with alternating light (summer) and dark (winter) layers

Glaciomarine Deposits

• Glaciomarine deposits result from glacier interaction with marine environments
  • Include ice-rafted debris dropped from icebergs
  • Subaqueous outwash fans form where meltwater enters the ocean
• Often contain a mix of terrestrial and marine sediments
• Can provide evidence of sea level changes and ice sheet extent

Glacial Deposition Processes

Basal Deposition

• Lodgment till forms when debris-rich basal ice freezes onto the glacier bed or clasts pushed into soft sediments beneath the glacier
• Melt-out till deposits when debris-rich ice melts slowly preserving some original structure of debris within ice
• Subglacial deformation till results from the shearing and mixing of sediments beneath moving ice

Supraglacial and Ice-Marginal Deposition

• Supraglacial deposition occurs when debris on glacier surface lowers onto landscape as ice melts often resulting in hummocky topography
• Ice-contact deposition happens when sediments deposit against stagnant or slow-moving ice resulting in features like kames and eskers
• Lateral moraines form from the accumulation of debris along glacier margins

Glaciofluvial and Glaciolacustrine Deposition

• Glaciofluvial deposition occurs when meltwater streams transport and deposit sediments sorting them based on particle size and flow velocity
  • Results in stratified deposits with varying grain sizes
  • Forms features like outwash plains, valley trains, and braided river systems
• Glaciolacustrine deposition involves suspended sediments settling in proglacial lakes often forming varves through seasonal variations in sediment input
  • Coarser sediments deposit near lake inlets
  • Finer sediments settle in deeper, quieter parts of the lake

Glacial Sedimentary Structures

Ice-Related Structures

• Dropstones appear as oversized clasts in fine-grained glaciolacustrine or glaciomarine sediments indicating ice-rafting processes
• Clast fabric refers to elongated clast orientation within till indicating ice flow direction and depositional processes
• Glaciotectonic features show deformation structures in sediments caused by glacial pressure
  • Include thrust faults, folds, and shear zones
  • Provide evidence of ice dynamics and subglacial deformation

Periglacial Structures

• Frost wedges and ice-wedge casts form by repeated freezing and thawing cycles in permafrost environments
• Cryoturbation structures result from frost heaving and soil mixing in active layer above permafrost
• Patterned ground develops in periglacial environments due to freeze-thaw cycles (stone circles, polygons)

Meltwater-Related Structures

• Kettles form as depressions from melting buried ice blocks often filled with water to form kettle lakes
• Eskers exhibit internal sedimentary structures reflecting subglacial stream deposition
  • Include cross-bedding, ripple marks, and graded bedding
• Kame terraces show ice-contact stratified drift deposited between valley walls and glacier margins

Glacial Deposits for Paleo-Reconstruction

Sediment Analysis Techniques

• Sediment composition and provenance analysis reveal source areas of glacial deposits and ice flow pathways
• Clast shape and roundness provide information on transport distances and mechanisms
  • More rounded clasts indicate longer transport or reworking
• Grain size distribution in glacial deposits reflects depositional environment and energy conditions during deposition
• Fabric analysis of elongated clasts in till indicates ice flow direction

Landform and Structure Interpretation

• Sedimentary structures like cross-bedding in glaciofluvial deposits indicate paleocurrent directions and flow regimes
• Glaciotectonic structures provide evidence of ice dynamics
  • Include ice thickness, flow direction, and deformation of underlying sediments
• Spatial distribution and geometry of glacial landforms reconstruct ice sheet extent, retreat patterns, and glacial lake evolution
• Moraines mark former ice margins and can be used to map glacier retreat stages

Chronology and Climate Reconstruction

• Absolute dating techniques applied to glacial deposits allow reconstruction of glacial chronologies
  • Include radiocarbon dating, optically stimulated luminescence, and cosmogenic nuclide dating
• Relative dating methods help establish sequences of glacial events
  • Include weathering rind thickness, soil development, and lichenometry
• Analysis of plant and animal remains in glacial deposits provides information on past climate conditions
• Stable isotope analysis of ice cores and glacial sediments reveals temperature and precipitation patterns