P3 Flashcards
Fluvioglacial Processes and Landforms:
Are created by the meltwater from glaciers, largely through deposition but also by erosion
• Discharge (predominantly in summer months) occurs through supraglacial and subglacial streams at the base of the glacier (often under a great deal of pressure in the latter case)
Outwash Plains (Sandur):
Comprised of gravels, sands and clays deposited by meltwater streams
• The plains tend to sort the sediment so that coarser material, like sand, is closer to the snout, and finer clay is furthest away. (Outwash material may also be deposited on top of till following the retreat
Varves:
- Are glacio-lacustrine sediments deposited annually in lakes at the glacial margins.
- Varved clays exhibit alternating layers of darker-coloured silt on top of layers of lighter-coloured sand.
- The larger-calibre materials (light) are deposited during late spring when meltwater streams experience peak discharge and maximum load.
- In the cooler autumn, when volumes of meltwater decrease, streams experience lower discharge and can only carry finer-calibre sediments so only they (darker coloured silts) are deposited.
- This process repeats each year and so such bands are formed.
- By counting the number of varves it is possible to obtain the age of the sediment.
- Thicker or thinner varves may indicate warmer periods (more meltwater) and colder periods with less
meltwater.
Braided Streams:
• Seasonal variation in the discharge of meltwater streams leads to fluctuations in the sediment load being carried across the outwash plains.
• Deposition of excess sediment during times of lower discharge may obstruct flow, leading to braiding of the channels as water seeks to find a more efficient way through.
• An example is the River Eyra in Iceland.
Eskers:
Description:
- Sinuous ridges which mark the course of streams that once transported subglacial meltwater beneath the glacier
- May run uphill (as meltwater streams can under hydrostatic pressure)
- Consist of sorted silt, sand and gravel - rounded due to rivers vigorous attrition
- Form at right angles to the ice front/run parallel to the ice flow
- Some degree of sorting both downstream and from the centre outwards
- Vegetation on the esker may differ to that nearby
- Generally develop in an intricate network rather than in isolation
- Are often beaded due to stationary ice phases They reveal major routes followed by meltwater
- Slopes/sides are 5-10 degrees steep
- Examples are: Trim Eskers in Goltrim near Dublin, Ireland
Formation of eskers
- Four conditions are necessary for the formation:
- Running water
- High amounts of transported sediment
- A change in the velocity of flow, resulting in deposition
Retreat of the ice margin to expose the esker - Beneath a glacier or ice sheet, meltwater flows through a network of tunnels, often under hydrostatic pressure.
- The pressure is so great in places that the water is even forced up gentle gradients.
- When the discharge is high, a lot of sediment is transported.
- However, when the discharge falls at the end of the summer melt season, sediment is quickly deposited.
- Then subglacial or englacial tunnels fill with sediment, which is exposed when the ice margin retreats.
- There are three types of esker:
There are three types of esker:
- Sharp-crested eskers; debris is deposited along the sides of the channel where the flow velocity is the lowest. Preferential melting of the crest of the channel carves out the top.
- Multiple-crested eskers; where the topography of the bedrock influences the location of the esker, the channel may migrate downslope. Debris is deposited as the esker moves and two or more crests are formed on the esker.
- Broad-crested eskers; water will re-freeze in the channel if the energy of the water cannot keep the temperature above freezing. The channel will broaden because only the parts of the channel that are in contact with the flowing water will not freeze.
Kames:
Description:
• A Kame is a mound of sorted sands and gravels deposited by meltwater
•
Usually layered
• Max width of 50m and height of approximately 3-5m
• Examples:
• Cairngorms in Scotland
North Norfolk near the Blakeney esker 20-400m diameter and up to 20m high
Kame Formation theory 1:
• Along the front of a stationary or slowly receding glacier.
• Streams flowing off the ice build up a small delta in the static water if an ice marginal lake
• One side of the sediment remains in contact with the ice and during deglaciation the ice melts, the support is removed and the sediment slumps
Kame Formation theory 2:
(Homes in 1947)
Pools or ponds develop on the surface or within stagnating ice as meltwater slows into depressions
The sediment builds up into a mound and as the ice around the deposit melts, the kames are lowered and deposited at the base of the ice
Kame Terraces:
Description:
• Elongated mounds of sorted sand and gravel.
• Found at the sides of glaciers/former glaciers.
Kame terraces Formation:
- From sediment deposited by meltwater in contact with the ice margin.
- Form at the sides of glaciers.
- Heat from the sun warms the valley sides and melts the ice that is in contact with them.
- During the summer, meltwater runs along the edge of the glacier and can form marginal lakes.
- At times of low meltwater flow, material is deposited in layers, fining upwards.
- In this way the marginal lakes are infilled.
- The sediment is well bedded and comprises sorted and gravel
- This material remains in contact with the ice until the glacier wastes downwards, leaving a terrace
stranded at the side of the valley.
Kettles or Kettle Holes:
Description:
- Hollows found in sandur plains or vast till plains
- Often filled with a pond or marshy ground
- Between 5-100m diameter, 10-50m deep
Kettles or Kettle Holes Formation::
• As an ice sheet retreats, some blocks of ice may become detached (known as dead ice)
• Often dead ice becomes embedded into the sandur plain and is covered with sediment
• As the climate warms, the dead ice melts and leaves a depression in the surface
• Surrounding settlement is meanwhile made unstable and collapses into the pit
• May afterwards be filled by a pond or marshy ground
