Glacial Deposition A2

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Information about Glacial Deposition A2

Published on October 16, 2007

Author: RCha

Source: slideshare.net

Material deposited by glaciers is collectively termed GLACIAL DRIFT – It can be sub-categorised into: DIRECT DEPOSITION TILL - Glacial deposits, unstratified and unsorted material (boulder clay) – large fragments of rock of all sizes and shapes randomly mixed together. Can be divided into: 1. lodgement till – deposited at base of glacier due to basal melting – material is smeared onto the underlying rock due to the pressure of the ice as it continues to move 2. ablation till – material deposited as the ice melts away (coarser than lodgement till 2. INDIRECT DEPOSITION a. FLUVIOGLACIAL – picks up load once carried by ice (often then carried beyond limits of glacier – sorted and stratified by the action of water b. ICE – CONTACT STRATIFIED DRIFT – partly sorted by water – roughly stratified – deposited in vicinity of melting ice. GLACIAL DEPOSITION

Features of Glacial Deposition: Till Deposits Moraines Drumlins Till Plains Erratics

Moraines

Drumlins

Till Plains

Erratics

A DRUMLIN

DRUMLINS Characteristics: Smooth elongated mounds of till – long axis parallel to the direction of the ice movement (mounds of glacial debris streamlined into elongated hills) Where found in clusters – called “ drumlin swarm” – classic ‘basket of eggs topography’ Range in size from small mounds (2m high / 10m long) to huge hills – can be over a km long and 100m in height. shape of drumlin measured using elongation ratio = Length of drumlin max width Usually between 25:1 and 4:1 – greater elongation suggests more powerful ice flow.

Characteristics:

Smooth elongated mounds of till – long axis parallel to the direction of the ice movement (mounds of glacial debris streamlined into elongated hills)

Where found in clusters – called “ drumlin swarm” – classic ‘basket of eggs topography’

Range in size from small mounds (2m high / 10m long) to huge hills – can be over a km long and 100m in height.

shape of drumlin measured using elongation ratio =

Length of drumlin

max width

Usually between 25:1 and 4:1 – greater elongation suggests more powerful ice flow.

A Drumlin Field – also known as a Drumlin Swarm – typical “Basket of Eggs” topography

Western Alberta, Canade Theories of formation: There are several theories for the formation of drumlins – and it is believed that different drumlins probably form in different ways (our understanding is incomplete. The formation is likely to include one or more of the following: Formed as ice become overloaded – resulting in lodgement of subglacial debris as it melted out of the basal ice layers and further ice movement streamlines and moulds the material; As glacier re-advances, previously deposited material is re-shaped Accumulation of material around a bedrock obstruction (rock-cored drumlins); Thinning of ice as is spreads out over lowlands resulting in reduction in competence of glacier – debris deposited and streamlined by continuing forward movement; some believe catastophic floods beneath ice sheets ripple ground moraines into mounds / hollows. Examples of Drumlins : Hellifield, Ribblesdale – North Yorkshire – Swarms 40m high, 50-500m long – may be the result of ice sheets coming down from the Lake District, overloaded – change of gradient on the lowlands may have caused the ice to deposit the drumlins here. Other examples: New York State (USA) – largest drumlin field – 10,000 drumlins! Eden Valley - Cumbria

Theories of formation:

There are several theories for the formation of drumlins – and it is believed that different drumlins probably form in different ways (our understanding is incomplete. The formation is likely to include one or more of the following:

Formed as ice become overloaded – resulting in lodgement of subglacial debris as it melted out of the basal ice layers and further ice movement streamlines and moulds the material;

As glacier re-advances, previously deposited material is re-shaped

Accumulation of material around a bedrock obstruction (rock-cored drumlins);

Thinning of ice as is spreads out over lowlands resulting in reduction in competence of glacier – debris deposited and streamlined by continuing forward movement;

some believe catastophic floods beneath ice sheets ripple ground moraines into mounds / hollows.

Stoss End Lee Slope Direction of Ice Flow

Lateral Moraine Formed from debris frost shattered from the valley slopes – material falls onto surface of glacier and is carried along its edges. - when melting takes place, an embankment of material is left against the valley sides. MORAINES

Formed from debris frost shattered from the valley slopes – material falls onto surface of glacier and is carried along its edges.

- when melting takes place, an embankment of material is left against the valley sides.

Medial Moraine Formed where two glaciers meet – resulting in the mergence of two lateral moraines. material is often supraglacial and only about 1m of coarse debris therefore, they rarely give signficant landforms in post-glacial periods.

Formed where two glaciers meet – resulting in the mergence of two lateral moraines.

material is often supraglacial and only about 1m of coarse debris

therefore, they rarely give signficant landforms in post-glacial periods.

Terminal Moraine This marks the max. extent of the glacier, forming at the snout. - resembles large mound of debris (typical arc shaped) – shape dependent on: 1. amount of material carried 2. rate of ice movement 3. rate of ablation Few glaciers today have terminal moraine in contact with ice – due to rapid glacial retreat. - terminal moraine, marks the boundary between glacial / proglacial areas (unsorted / angular) material.

This marks the max. extent of the glacier, forming at the snout.

- resembles large mound of debris (typical arc shaped) – shape dependent on:

1. amount of material carried

2. rate of ice movement

3. rate of ablation

Few glaciers today have terminal moraine in contact with ice – due to rapid glacial retreat.

- terminal moraine, marks the boundary between glacial / proglacial areas (unsorted / angular) material.

Recessional Moraine Deposited by glacier as it retreats (if glacier stays still long enough for mound to build up a mount of material) longer the pause – greater size of mound; parallel to terminal moraine (90o to direction of movement) - if there are several periods of retreat, there may be small sub-parallel ridges.

Deposited by glacier as it retreats (if glacier stays still long enough for mound to build up a mount of material)

longer the pause – greater size of mound;

parallel to terminal moraine (90o to direction of movement)

- if there are several periods of retreat, there may be small sub-parallel ridges.

Push Moraine If the glacier readvances (e.g. due to climate deterioration) – previously deposited material may be shunted back up. - this can be recognised by individual stones which are pushed upwards from their original horizontal position.

If the glacier readvances (e.g. due to climate deterioration) – previously deposited material may be shunted back up.

- this can be recognised by individual stones which are pushed upwards from their original horizontal position.

ERRATICS CHARACTERISTICS: These are pieces of rock that geologically are out of place They can vary from small pebbles to huge boulders. FORMATION: These rocks were initially supra-glacial debris (either from plucking or from rockfall from weathered slopes above the glacier. They have then been transported and deposited into an area of differing rock type (hence the geological difference) EXAMPLES: Norber Erratics (Yorkshire Dales) – see photo – Blocks of Silurian shale – deposited on carboniferous limestone – erratics have protected the underlying rock from carbonation weathering, resulting in the erratics being perched on pedestal’s of the underlying rock. Bluish Granite blocks unique to the island of Ailsa Craig (Ayrshire) found on the SW Lancashire Plain – shows the direction of ice flow and a journey of at least 240km; Chalk Rafts – transported from the North Sea bed to West Runton

CHARACTERISTICS:

These are pieces of rock that geologically are out of place

They can vary from small pebbles to huge boulders.

FORMATION:

These rocks were initially supra-glacial debris (either from plucking or from rockfall from weathered slopes above the glacier.

They have then been transported and deposited into an area of differing rock type (hence the geological difference)

EXAMPLES:

Norber Erratics (Yorkshire Dales) – see photo – Blocks of Silurian shale – deposited on carboniferous limestone – erratics have protected the underlying rock from carbonation weathering, resulting in the erratics being perched on pedestal’s of the underlying rock.

Bluish Granite blocks unique to the island of Ailsa Craig (Ayrshire) found on the SW

Lancashire Plain – shows the direction of ice flow and a journey of at least 240km;

Chalk Rafts – transported from the North Sea bed to West Runton

TILL PLAINS Formed when large masses of unstratified drift, deposited at the end of an advance, smother the surface. It is an unsorted mixture of rocks, clays and sands. How transported? Largely transported as supraglacial debris – later deposited to form moraine – either during periods of active ice movement or when glacier was in retreat . Description? Individual stones are sub-angular – not rounded live river material and don’t posses the sharp edges of rocks recently broken up by frost-shattering; Composition? Composition of till reflects the character of the rocks over which it has passed – e.g. East Anglia – covered by chalky till – as the ice passed over a chalk escarpment.

Formed when large masses of unstratified drift, deposited at the end of an advance, smother the surface. It is an unsorted mixture of rocks, clays and sands.

How transported?

Largely transported as supraglacial debris – later deposited to form moraine – either during periods of active ice movement or when glacier was in retreat .

Description?

Individual stones are sub-angular – not rounded live river material and don’t posses the sharp edges of rocks recently broken up by frost-shattering;

Composition?

Composition of till reflects the character of the rocks over which it has passed – e.g. East Anglia – covered by chalky till – as the ice passed over a chalk escarpment.

Till Fabric Analysis What is it? This is a fieldwork technique used to determine the direction and source of glacial deposits Involves measuring the orientation and dip of the long axes of a sample of 50-100 stones within the till – this is then plotted on a rose diagram Stones and pebbles carried by glacier – tend to become aligned with their long axes parallel to the direction of ice flow – offers least resistance to the ice. However – slumping; freeze-thaw and later ice-advances may disturb sediments in which case this type of analysis will not enable the reconstruction of the original direction of ice movement. Reference: All photographs used for educational use only – sourced from google images.

Till Fabric Analysis

What is it?

This is a fieldwork technique used to determine the direction and source of glacial deposits

Involves measuring the orientation and dip of the long axes of a sample of 50-100 stones within the till – this is then plotted on a rose diagram

Stones and pebbles carried by glacier – tend to become aligned with their long axes parallel to the direction of ice flow – offers least resistance to the ice.

However – slumping; freeze-thaw and later ice-advances may disturb sediments in which case this type of analysis will not enable the reconstruction of the original direction of ice movement.

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