Glacial Geology

Three types of glacial erosion:

1. Abrasion
2. Quarrying
3. Subglacial water

ABRASION

• Base of a glacier contains debris (“tools”; clay to boulder sizes)
• Sliding glacier drags debris across the bed surface
• Abrades (i.e., scratches/carves) if debris is harder than the bed
surface

ABRASION MODEL

If U i > U m (sliding rate > melting rate) the tool remains in motion
and abrades the substrate

Debris concentration in basal ice (Cr)

QUARRYING

• Rock is quarried from the bed by the glacier
• Produces coarse material used for abrasion
• Produces locally roughened bed

DEBRIS ENTRAINMENT

• Supraglacial – valley walls
• Important for valley glaciers
• Not relevant to ice sheets
• Subglacial – from substrate
• Removed from the bed

Regelation

Melting and refreezing ice around subglacial bumps

Freeze-on

Debris freezes to the ice and is transported away
Two types for debris entrainment:
1. Hydraulic jacking
2. Supercooling

GLACIAL TRANSPORT

1. Regelation
2. Freeze-on

USPENDED LOAD – supraglacial, englacial

Particles move with ice
U p = Ui

TRACTION LOAD – subglacial

Particles move at the base
U p < U

Clast shape:

• Intermediate/Long (dI/dL)
• Short/Intermediate (dS/dI)

• Shales & limestones = rods, discs
• Granites & gabbros = spheres

Abrasion removes angular corners and produces faceted surfaces

GLACIAL DEPOSITION

Occurs by a number of different processes

Results in a number of different facies

• Poorly sorted to unsorted
• Can range in grain size from clay to boulders

Processes:
1. Lodgement
2. Meltout
3. Sediment deformation
4. Gravity (subaerial)
5. Gravity (subaqueous)

Diamicton

nongenetic term for such sediments

Till

diamicton deposited directly by glacial ice

basal melt-out till

• Gradual in-situ melting of debris-laden
basal ice
• Released from non-sliding,
non-deforming ice
• No mixing or deformation of debris

Basal melt-out from stagnant ice, where
either

BASAL MELT-OUT TILL Characteristics:

• Passive process – till retains characteristics of debris in transport
• Melting of foliated ice (debris layers separated by clean ice)

• Fabrics – preferred upglacier dip, but dip may settle to lower angle after melting

• Matrix-supported, poorly sorted
• Strain increases upward from base of deforming layer

RESEDIMENTATION

• Debris flows in ice-contact proglacial environments
• Deformation occurs when 𝜏 > 𝜏s

RESEDIMENTATION-characteristic

• Erosion occurs at the base where strain rate is greatest
• Inverse grading – reduced 𝜏 s due to deformation leads to loss of support for
larger clasts
• Stacked interbedded flows from multiple sources with variable fabric directions

GLACIAL LANDFORMS

• Defined by physical characteristics:
• Elevation
• Slope
• Shape
• Orientation

EROSIONAL LANDFORMS

Types of features:
• Striae
• Roches moutonnées
• Fjords/troughs
• Cirques

Inform:
• Location
• Presence and type of glacier
• Orientation
• Thermal regime

STRIAE

• Product of subglacial abrasion
• Scoring of particles creates linear gouges in rock surface

ROCHE MOUTONNÉE

• Asymmetric bedrock bumps or hills
• Stoss & lee sides
• Combination of abrasion (up-ice stoss side) and quarrying (down-ice lee side)
under sliding ice

FJORDS/TROUGHS

• Carved by topographically-restricted ice flow through rock channels

• Fjords are flooded by marine waters
• Troughs are not

Formation of U-shaped valley:

• Cross–sectional velocity profile of a glacier decreases radially from highest at center surface
• Erosion greatest at highest sliding velocities
• Initially, highest sliding velocities part way up valley
• Broadening & steepening of valley
• Downcutting after subglacial topography matches velocity profile
• Zone of glacier influence = oversteepening and collapse of cliffs due to downcutting glacier
• Erosion rates higher in troughs than their tributary glaciers
• Creates hanging valleys

CIRQUES

• Flat-floored basins connected to a steep backwall by a concave slope
• “Amphitheater”–shaped depression formed on side of a mountain
• Only found in mountainous terrain

CIRQUE FORMATION

• Pre-existing hollow in mountainside from fluvial erosion or upslope failures
• Glacier ice occupies the hollow
• Thick & steep enough to generate stresses for ice-deformation & sliding

Classify depositional landforms with environmental associations:

1. Subglacial: Drumlins & eskers
2. Ice-marginal: Terminal & lateral moraines
3. Supraglacial: Medial moraines, kames, & kettles
4. Proglacial: Sandar
5. Glaciolacustrine & glaciomarine: Grounding line & subaqueous moraines

DRUMLINS

• Elongate, streamlined ridges of sediment aligned parallel to ice-flow
“inverted spoon or egg half-buried along its long axis”
• Stoss & lee sides

• Drumlins tend to concentrate in “fields” with hundreds to thousands present
• Mostly composed of unconsolidated sediment

ESKERS

• Elongate, sinuous ridges of glaciofluvial sand and gravel
• Can be 100’s of km long & several km wide
• In-filling of ice-walled river channels (sub-, en-, or supraglacial)• Planform of • Planform of eskers as variable as fluvial systems

MORAINES

• Aprons of unstratified sediments deposited along the margins of a glacier/ice sheet
• Mark the present and/or former margins

Classification of moraines

• Terminal moraine – the outermost moraine formed at the max. limit of advance
• Recessional moraine – younger moraines found “upvalley” of terminal moraine
• Lateral moraine - formed along the lateral aspects (i.e., sides) of a glacier or ice sheet

MORAINES Morphology & composition

• Unconsolidated & unstratified diamictons
• Wide-ranging clast sizes
• Deposited through resedimentation or subglacial sediment deformation
• Sharp crested ridges
• Large range of sizes and heights

MEDIAL MORAINES

• Supraglacial sediments formed at the confluence of two glaciers
• Poor preservation potential
• May be found as low-relief boulder lines

KAMES

• Mounds or hill of glaciofluvial deposits
• Reworked supraglacial debris during glacier wastage (calving, melting, etc.)

Kettles

• Depressions created by land subsidence after melting of buried ice
• May be filled with water (kettle lake)

SANDAR

• Flat plains of glaciofluvial outwash sediments
• Decrease in mean grain size further from glacier terminus
• Braided fluvial system & facies
• Discharge (Q) a function of diurnal & seasonal meltwater runoff

GLACIAL CHRONOLOGY

• Dating of landforms to determine the timing of glacier activity

• Multiple geochronologic methods, but two dominant ones in glacial geology:
• Radiocarbon
• Cosmogenic surface exposure dating

GALACTIC COSMIC RADIATION

Limitations on GCR reaching Earth’s surface:
• Solar winds
• Increased solar activity reduces GCR flux
• GCR flux inversely related to number of sunspots
• Geomagnetic field
• GCR with vertical incidence deflected away by horizontal component of magnetic field
• Horizontal component highest at low latitudes & lowest at high latitudes
• Atmospheric density
• GCR collide with nuclei in upper atmosphere to create cascading “shower” of high-energy secondary cosmic rays
• Attenuation of cosmic rays by increased atmospheric density

RADIOCARBON DATING

• Application: 0 - 50,000 years
• Main isotopes of C (relative abundance):
• 12C (98.89%)
• 13C (1.11%)
• 14C (10 -12%

• Produced through interaction between cosmic rays
and nitrogen

SURFACE EXPOSURE DATING

• Date the time since first exposure of surface materials
• Common nuclides used for exposure dating

• Produced through the interaction of cosmic rays and target elements
• Factors that affect production on Earth at any given time

Spallation

Neutron collides with target element & splits nucleus into several lighter particles – most common

Muon capture

Muons (“heavy electrons”) combine with nucleus to transform a proton into a neutron – small contribution

Factors affecting nuclide accumulation

• Variations in cosmic ray flux
• Shielding by surrounding topography
• Self-shielding
• Erosion and exhumation
• Snow, vegetation cover history
• Nuclides accumulated from prior exposure

He, Be, Al, Cl