7.3 Glacial deposition and sedimentary structures
4 min read•july 30, 2024
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
Top images from around the web for Till and Moraines
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
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
- 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 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
- 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
- deposition occurs when debris on glacier surface lowers onto landscape as ice melts often resulting in hummocky topography
- 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
Key Terms to Review (18)
Abrasion: Abrasion is the process of wearing away surfaces through friction or impact caused by particles or other materials. This physical weathering mechanism plays a significant role in shaping landscapes and is a key component in various erosional processes, influencing landforms and sedimentation across different environments.
Drumlin: A drumlin is a streamlined, elongated hill formed by glacial deposits, typically composed of till, that occurs in a landscape shaped by past glacial activity. These features are often found in groups, indicating the direction of glacial movement and serving as important indicators of the dynamics of ice sheets and the sedimentary processes involved in glacial environments.
Glacial erosion: Glacial erosion is the process by which glaciers shape the landscape by wearing away rocks and soil as they move. This dynamic interaction between ice and the earth's surface leads to various landforms and geological features, profoundly influencing topography and sediment distribution. Understanding this process highlights how glaciers act as powerful agents of change, carving valleys and transporting sediments over large distances.
Glacial till: Glacial till is unsorted and unstratified sediment deposited directly by a glacier as it advances or retreats. This type of sediment consists of a mix of rock fragments, clay, silt, sand, and boulders, varying greatly in size and composition. The presence of glacial till provides important clues about past glacial movements and the conditions under which they formed.
Glaciofluvial: Glaciofluvial refers to the processes and landforms associated with the movement of meltwater from glaciers, which transports and deposits sediment as it flows. This term connects closely to the dynamics of glacial deposition and sedimentary structures, as glaciofluvial environments often exhibit unique features created by the interaction between ice, water, and sediment. These features play a significant role in understanding how glaciers influence the landscape and shape geological formations.
Glaciomarine: Glaciomarine refers to sedimentary environments that are influenced by both glacial and marine processes, typically occurring where glaciers meet the sea. This term captures the complex interplay between ice, water, and sediment transport, resulting in unique depositional features and sedimentary structures formed by melting glaciers and marine dynamics.
Ice-contact: Ice-contact refers to the interactions and processes that occur when glacial ice is in direct contact with the underlying substrate, which can significantly influence sediment deposition and landscape formation. This contact leads to unique depositional features such as moraines and till, shaped by the ice's movement and melting patterns. Understanding ice-contact is crucial in recognizing how glaciers transport and deposit materials in their wake.
Kettle lake: A kettle lake is a body of water that forms in depressions left behind by melting glaciers. These lakes are typically round or oval-shaped and can vary greatly in size. They play an important role in shaping local ecosystems, as they often provide habitats for various plant and animal species, while also influencing groundwater recharge and surface water dynamics.
Moraine: A moraine is a landform created from the accumulation of debris, primarily composed of rocks and sediments, that has been transported and deposited by glaciers. Moraines serve as important indicators of past glacial activity and are essential in understanding how glaciers interact with the landscape, shaping it through both erosion and deposition processes.
Outwash plain: An outwash plain is a flat, sediment-rich area formed by the deposition of materials transported by meltwater from glaciers. These plains are characterized by layers of sand and gravel that have been washed out from the glacier and spread out over the surrounding landscape. The outwash plain is an important landform that showcases the processes of glacial deposition, playing a significant role in shaping the environment during and after glaciation events.
Paleoglaciology: Paleoglaciology is the study of ancient glaciers and ice sheets, focusing on their formation, movement, and impact on the Earth's surface over time. This field of study helps scientists understand past climate conditions and the processes that shaped landscapes, providing crucial insights into glacial deposition and sedimentary structures left behind as glaciers retreated.
Plucking: Plucking is a glacial erosion process where a glacier removes and incorporates rocks and sediment from the bedrock as it moves. This process occurs when the glacier freezes onto the underlying rock and, as it advances, it pulls pieces of the rock away, effectively eroding the surface. Plucking is crucial in shaping landscapes, contributing to distinctive landforms such as U-shaped valleys and jagged mountain peaks.
Sandstone: Sandstone is a clastic sedimentary rock composed mainly of sand-sized mineral particles or rock fragments, typically cemented together by minerals like silica or calcite. This rock type forms in various environments, including deserts, riverbeds, and beaches, and can also play a significant role in the understanding of glacial deposition and sedimentary structures due to its ability to preserve the history of past geological processes.
Striations: Striations are linear grooves or scratches found on rock surfaces, formed by the movement of glaciers over bedrock. These features provide important evidence of past glacial activity, indicating the direction of glacier movement and the nature of the underlying material. Striations are crucial for understanding glacial erosion processes and can help reconstruct ancient environments shaped by ice.
Subglacial: Subglacial refers to the environment and processes occurring beneath a glacier or ice sheet. This area plays a crucial role in glacial dynamics, influencing how glaciers move, how they deposit sediments, and how they interact with the underlying bedrock. Understanding subglacial processes is key to comprehending glacial deposition and the formation of various sedimentary structures associated with glaciers.
Supraglacial: Supraglacial refers to the environment or processes occurring on the surface of a glacier. This area plays a critical role in glacial dynamics as it is where meltwater, debris, and sediment are often found, significantly influencing the glacier's movement and stability. Understanding supraglacial features is essential for studying glacial deposition, as these surfaces can affect how and where sediments are transported and deposited.
Valley glacier: A valley glacier is a type of glacier that forms in mountainous regions and flows down valleys, carving out U-shaped valleys and other distinctive landforms. These glaciers are typically confined by the surrounding terrain, and their movement is influenced by gravity and the underlying topography, making them crucial in shaping the landscape of mountain ranges.
Varved clay: Varved clay is a type of sedimentary deposit characterized by alternating layers of fine-grained sediment, typically clay and silt, that represent annual cycles of deposition. Each pair of layers typically consists of a light-colored summer layer, which is coarser and thicker, and a darker, finer winter layer, reflecting the seasonal changes in sediment supply due to glacial melting and freezing cycles. This unique layering helps in understanding past climate conditions and glacial activity.