Periglacial processes Periglacial processes and landformsand landforms

Pgs 145-151Pgs 145-151

‘Peri’ comes from the Greek meaning round or about

Therefore,

periglacial is applied to areas at or around the fringes of glaciers

Occur in areas where there is frost action such as freeze-thaw and frozen ground

Periglacial zone has no fixed location because of frequent and major ice advances and retreats over the last 2 million years – constant change

S. and E. England fossil landscape features formed by periglacial processes can be recognized as there was no later ice advances to destroy them

Today… periglacial processes within British Isle’s are restricted to tops of higher mountains

Worlds main periglacial zone covers Alaska and Artic lands of Canada and Russia

E

PermafrostPermafrost It means permanently frozen.

Permafrost itself is not a process, it’s a ground condition. Bedrock, regolith, and soil are frozen, between these layers are filled with ice.

Porous rock (chalk) is turned into impermeable rock/mass by permafrost.

Weathered rock and soil are cemented together and become solid.

In winter months permafrost can be up to 600m deep.

Permafrost has greater presence on mountains and ridges compared to river valleys.

Active layer in shallow surface zone defrosts in summer.

Water released is unable to drain away as ground beneath is frozen. Upper soil becomes saturated and can move on slopes of as little as 2°-Solifluction.

In Autumn refreezing begins on surface where heat is lost first.

As active layer narrows, expansion of water above and below causes surface heaving and associated landforms layer, eg patterned ground.

D

Mass movementMass movement

There are several factors affecting mass movement:

- The ground freezes and re-freezes throughout a year.

This leads to water being trapped above the permafrost.

- The temperature isn’t high enough for the water to evaporate.

- There is little vegetation to hold the soil together

The ground is often on slopes

L

Mass Movement Cont…Mass Movement Cont… Solifluction = important periglacial process as widespread occurrence

Definition = when the active later thaws in summer, excessive lubrication reduces the friction between soil particles – leads to solifluction or lobes

Appearance = these are rounded, tongue-like features often forming terraces on the sides of valleys

Effect = to reduce and smooth out the relief as weathered and loose deposited materials are transferred from upper slope to downslope

Therefore, there are more features at the foot of the slope:

E

Mass Movement ContMass Movement Cont The formation of lobes and terraces gives it a greater form

by making the lower slopes rounder and flatter areas:

There are two types of lobes

These are both controlled by the amount of vegetation

present:

1. Lack of vegetation cover, stone-banked lobes/terrace formed:

Small-lobes – rising up to 5m then extending up slope to form terrace between 10-30 metres long

Distinctive feature – each lobe is the stone wall behind the finer material

Stones on surface travel quicker downslope that finer materials until something changes the movement, e.g. vegetation

Finer material accumulates behind the wall to form a terrace

D

Mass Movement ContMass Movement Cont2. Forms under continuous vegetation cover where frost heaving is less effective at

raising stones to surface

Turf-banked lobes and terraces form instead, as it is the surface layers of turf and topsoil which are moved by solifluction

Terraces are created where minor relief obstructs movement, rolling up the turn flow backwards under continued surface movement

= forms as the riser below the lobe

When held together by vegetation and roots, lobes usually and cover larger areas on lower slopes then stone-banked areas

L

Mass Movement Finally!!!Mass Movement Finally!!! Fossil solifluction deposits are the most widespread periglacial relict feature in the British Isles General term for this = Head deposits

Widespread below outcrop of granite on Dartmoor and elsewhere in S.W England

Coombe Rock =

Name given to fans of chalk below escarpments and in dry valleys in down land regions in Southern England

Lobes/terraces identified in uplands of Scotland and Wales are exposed to actions of solifluction for the longest time in the British Isles

E

Frost Action and Resulting Frost Action and Resulting LandformsLandforms

Freeze-thaw is the most common weathering process in periglacial areas.

It brakes down the majority of rocks on the mountainsides and many of the landforms lower down.

L

ThermokarstThermokarst

Name given to very irregular surfaces of mostly hollows and small hummocks.

These pitted surfaces resemble those formed by solution in some karst areas of limestone.

Small domes that form on surface due to frost heaving with the onset of winter are only features.

They then collapse in summer thaw leaving small surface depressions.

Some ice lenses grow and form surface hummocks whish may last many years before they thaw.

D

PingosPingos

Small circular hills up to 50 or 60 m high

Occur in groups

Add to variety of thermokarst scenery

Pond in central crater makes even more distinctive

Greatest number found on flat lands in and around the Mackenzie Delta on Artic coast of N.W. Canada

Within each pingo is a large ice core, responsible for the frost heaving which forms the conical hill

After the lake forms, its ice core is no longer insulated against summer warmth – the gradient thaws away and the pingo form is lost

E

Patterned GroundPatterned Ground

Stones in periglacial areas seemed to sort themselves out in to patterns. This is due to frost heaving.

The ice freezes underneath the stones which expand and pushes the stones upwards.

When they reach the surface they roll outwards.

L

Other Periglacial Other Periglacial Processes Processes

and Landformsand Landforms•  Mechanical weathering by freeze-thaw is widespread.

•  Low temperatures, sparse vegetation cover and poorly developed soils reduce the likelihood of chemical and biological weathering taking place.

•  Lack of vegetation increases wind (aeolian) action.

•  Further erosion occurs as the wind-blown sand grains abrade rock outcrops.

•  Wind action produces semi-circular dunes of sandy outwash material.

•  It also produces wind-blown loess.

•  Loess is a fertile, easy to work soil.

•  A loess belt extends across Europe

C

Europe’s Loess BeltEurope’s Loess BeltC

Loess

•  The amount of transport and erosion in Arctic streams is also huge.

•  The high load comes from flowing over areas liberally covered by loose debris from glaciers.

•  The high discharge comes from melting ice.

•  80% of the discharge comes in one or two months of the year.

Artic StreamsArtic StreamsC

The End!The End!