Glacial deposits & periglacial landforms

Glacial transport

Glaciers can carry sediment of any size and, like a conveyor belt, transport it in the direction of flow, toward the toe

• supraglacial

• subglacial

• englacial

Glacial Deposits

Several different types of sediment can be deposited in glacial environments

Till = material that is deposited directly by the ice

• very poorly sorted, may contain clay to boulders

Moraines

Layers or ridges made of till

• lateral moraine

• terminal moraine

• medial moraine

Terminal moraines

Terminal moraines record the maximum extent of a glacier

How do terminal moraines inform retreat?

Medial Moraines

Form from the confluence of two glaciers

Striking example of laminar flow in glacial ice

As ice passes equilibrium line, debris in the ice concentrates at the surface.

This debris slows melting, leading to an elevation high at the medial moraine

Eskers

Eskers are a subglacial depositional feature created by channelized flow beneath the glacier

Water flows downhill… except when pressure gradients force it uphill

Drumlins

A subglacial depositional feature created from basal till

Often found in flatter regions where glacier flowed

Number in the thousands in many drumlin “fields”

• disrupted drainage; lakes, trellis network

Periglacial Landscapes

Those affected by frost and freezing temperatures

Processes are non-glacial

• can be at the fringes of glaciers and ice sheets

• but can also persist where there are no glaciers

Key: phase change (cycle) between water and ice

Permafrost

A layer of soil that maintains temperature below 0 year-round

• Note, does not have to be wet

When the mean annual surface temperature is below 0

Active layer is the top layer of the soil that is seasonally thawed

Permafrost base is the maximum depth of permafrost

Permafrost base

Thermal diffusion governs the temperature profile

• derived from assuming dT/dt=0, and integrating

Permafrost depth is linearly dependent on the mean annual surface temp

Active Layer

Can perform a similar modeling for active layer thickness

• sinusodial temperature + latent heat

Equilibrium permafrost geometries

Putting together, coldest regions have thickest permafrost and thinnest active layer

Warming temps are pushing permafrost profiles out of equilibrium

Segregation Ice

Water flow follows gradients

If the freezing front is advancing downwards from surface, water moves upwards to meet freezing front

This concentrates water in the soil column at ice lenses, which make up a feature called segregation ice

Ice volume can exced pore volume, increasing the total volume of soil column

Frost Heave

Increased Soil column volume is accommodated by raised ground surface

Infrastructure experiences this ground motion called frost heave

Upfreezing of stones

Another consequence of frost heave, the upward transport of stones to the ground surface

Creates sorted circles

An emergent sorting of sediments by grain size, from an initially unsorted mixed-grain size sediment column

Experiment

Lab experiments revealed the soil-pull mechanism

Once 0 C isotherm was approx 1/3 of the way through the clast, the adfreeze bond was strong enough to support the clast weight → clast moves upward with frost heave

During thawing, material slumps into void, preventing movement back to original position

Displacement rate is proportional to vertical length scale of object

• Larger clasts move towards surface

• objects like pilings and fence posts are especially susceptible to “frost jacking”

Formation of sorted circles

Relatively fine material heavesupward more than coarse material

Creates topographic gradient that results in creep of material at surface

• coarse material transports more readily via creep

It always comes back to diffusive hillslope transport

Ice wedges

Freezing soil contracts, creates tension that generates cracks

Melt in active layer migrates downward and freezes, expanding the crack

• tapering wedges of nearly pureice form within permafrost layer

Patterned ground

Model prediction: equilibrium reached when stresses cannot open new cracks - polygons 22m on average