Glacial Hydrology, An understanding of hydrostatic pressure, imbrication, sorting and stratification

1. Glacial Hydrology
Lower Sixth

2. Starter
• Describe the different ways by which material can be
transported by a glacier
• How can the characteristics of the material help in giving
us clues as to how it was transported
• Compare and contrast lodgement and ablation till.
• Describe how sub glacial moraines differ from marginal
moraines.

3. The Role of Meltwater
• As seen meltwater plays a vital role in erosion,
entrainment transportation and deposition
• It is also indirectly involved in the processes of:
• Plucking
• Abrasion
• However, it is pivotal in the process of basal sliding and
sub glacial bed formation
• The two main sources of meltwater are:
• Surface melting
• Basal melting

4. Surface Melting
• Contributes most of the supply
• Peaks in mid summer
• Only source of meltwater for cold glaciers
• Supraglacial streams form on the top
especially in the ablation zone, often fast
flowing and can plunge through
crevasses but more commonly a ‘moulin’
(Cylindrical tunnel like a pothole)

5. Movement through the Glacier
• As it moves it may re-freeze or contribute to more
melting
• It may then reach a subglacial supply

6. Basal Melting
• Normally occurs in warm based glaciers
• Flows under hydrostatic pressure
• Can excavate subglacial tunnels by cutting through
bedrock
• Emerges through portals (caves) at the glacier snout

7. Processes of Erosion
• Glacier streams behave similarly to normal streams away
from the glacier
• Within the glacier ‘fluvio-glacial’ streams operate
differently because of the high pressure and velocity
• It causes erosion of the bedrock by abrasion, and
chemical weathering of the glacier
• It can also lead to intense erosion as it exits the glacier as
during deglaciation ablation rates can be very high leading
to large discharge.

8. Processes of Deposition
• Meltwater deposits material subglacially, englacially and
supraglacially
• This material is called ‘Ice contact fluvio-glacial deposit’
• Where the material is deposited at or beyond the ice
margin it is known as ‘proglacial or outwash’

9. Characteristics of fluvio- glacial Deposits
• Proglacial material is generally sorted and stratified by
the action of the meltwater
• Whilst ice contact deposits tend to be unsorted and
unstratified
• Smoother and rounder
• Sorted horizontally
• as the larger material tends to be found nearer the glacier
snout
• Stratified vertically
• which reflects the annual/ seasonal sediment changes

10. Clast shape and Imbrication
• Clast shape
– The clast maybe more angular or show
evidence of rounding due to the processes of
attrition
• Imbrication
– The clast may have a preferred orientation
and dip caused by the strong currents
– Clasts are aligned in the direction of ice
movement and often dip upstream
• Sorting
– Seasonal variations in stream discharge sorts
the grains into layers of consistent size
THIS CAN BE TESTED IN THE FIELD

11. Landforms of meltwater Deposition
Glacial Sediment (DRIFT)
Glacial Deposits
Fluvio-glacial
deposits
Landforms:
Erractics
Drumlins
Moraines
Till Plain
Landforms
Kames
Kame Terraces
Eskers
Kettle holes
Braided streams
Varves
Outwash plain
Ice contact
landforms:
Kames
Kame Terraces
Eskers
Proglacial
landforms:
Kettle holes
Braided streams
Varves
Outwash plain

12. Landforms of fluvio-glacial
Deposits
• Description: Long Sinuous ridges on the valley floor
• Explanation Formed from material in subglacial
tunnels as supply of hydrostatic meltwater decreases
• Subglacial streams carry large amounts of rocks and
debris due to their high hydrostatic pressure inside
tunnels
• When the glacier retreats it is deposited at a
consistent rate and forms a ridge as shown. They are
also distinctive to glacial deposits as they are
stratified and sorted and often have distinct bedding
of the sediments
Ice contact Fluvio-glacial

14. Kames
• Description An undulating
mound of Fluvio-glacial sand
and gravel deposited on the
valley floor near the glacier
snout
• Explanation: Formed as
meltwater streams emerge onto
the outwash plain or proglacial
lakes at the glacier snout, their
velocity suddenly falls and
sediment is deposited
• Kames form in zones of melting
ice such as crevasses, moulins
and larger cavities
• They are often sorted
• They are distinctive as they
have an irregular pattern as
compared to drumlins which
Ice contact Fluvio-glacial

15. Kame Terrace
• Description: A flat linear deposit
of Fluvio-glacial sand and gravel
deposited along the valley sides
• Explanation: During the summer
the valley sides radiate heat,
melting the edges of the glacier
and forming meltwater streams
which deposit sediment
• When the glacier retreats, the
sediment will fall to the valley floor
forming kame terraces
• They often have sorted sand and
gravel
Ice contact Fluvio-glacial

16. Outwash Plain (Sandur)
• Description: A flat expanse of
Fluvio-glacial sediment in front of
glacier snout (proglacial)
• Explanation: As meltwater streams
emerge from the glacier and enter
lowland areas, they gradually lose
energy and are deposited
• They are size sorted by the water
runoff of the melting glacier with the
finest materials, like silt, being the
most distantly re-deposited, whereas
larger boulders are the closest to the
original terminus of the glacier
• (This is also a good example of
braiding)
Proglacial Fluvio-glacial

17. Kettle Holes
• Description: Small circular
depression often forming a lake
in an outwash plain
• Explanation: As the glacier
retreats, detached blocks of ice
remain in the outwash plain.
• Meltwater streams flow over the
ice covering them in deposits of
Fluvio-glacial debris eventually
the ice melts and the debris
subsides to form a depression
which often then fills with
meltwater forming a lake
Proglacial Fluvio-glacial

18. Kettle Holes - >>WATCH<<

19. Proglacial lakes
• Description: A lake formed in front of the
glacial snout
• Explanation: Formed either by the
damming action of a terminal or
recessional moraine during the retreat of a
melting glacier, or by meltwater trapped
against an ice sheet due to isostatic
depression of the crust around the ice
Proglacial Fluvio-glacial

20. Meltwater Channels
• Description: A narrow channel cut into
bedrock or deposits, either underneath
or along the front of the ice margin
• Explanation: Meltwater can erode deep
channels, even gorges as a result of
high hydrostatic pressure within the
glacier and their high sediment load.
• The have unique characteristics:
– Under hydrostatic pressure Under
hydrostatic pressure beneath the glacier
they are able to flow up hill and they are
often larger than post glacial streams
– Braiding of meltwater channels is common,
due to seasonal variations in discharge
Proglacial Fluvio-glacial

21. Material (Fabric)
• The till fabric is often moved into the direction of flow due
to be re-worked by the ice and basal meltwater this means
they can be tested in the field into the direction that the
rocks face
• This should highlight the direction that the ice flowed and
help with understanding the direction the glacier travelled.
• They following exercise will highlight this:

22. Turn to page 81 (Oxford)
• Using the data in figure 8 draw the following graphs
– Mean Sediment size against Distance from Snout
– Average sediment shape against Distance from Snout
• (This is two separate graphs)
• Analyze each graph and describe the relationships if any
seen
• Other than Student t test what other tests could be run on
these relationships?
7.10 is on page 299
7.5 is on page 303

23. N. America
• Position of the ice
sheets in North
America at 10,000 and
7,000 years before
present
• This allowed many fish
to cross drainage
divides