2-3-2-glacial-deposition-landforms-and-landscapes

Glacial Deposition

• When debris is deposited by the ice mass and not meltwater, it is called ice contact deposition

• During the warmer summer months, glaciers begin to melt, and glacial till is deposited on the valley floor or sides of a moving glacier

• Till is unsorted, irregular debris ranging from clay to stones to boulders of any size and shape

• Features of ice contact deposition include erratics, moraines, and drumlins

Erratics

• Erratics are random boulders of different sizes and types from the area where they are found

• There is no pattern to their deposition, and they look completely out of place on the landscape

• Glaciers pick up large rocks and carry them hundreds, sometimes thousands of kilometres from where they originate 

  • Erratics from Scandinavia have been found in boulder clay of the North East of England coastline

• Erratics are carried deep in the ice and do not erode the same as till at the edges of the glacier

• An example is the Great Stone of Fourstones, (Big Stone) on the moors of Tatham Fells, England

Moraines

• Unsorted glacial till that is deposited in mounds are called moraines

• Moraines are termed from their position on the glacier:

  • Terminal: Material deposited at the snout of the glacier

  • Lateral: Material is deposited along both sides of the glacier

  • Medial: Ridge of deposited material in the middle where two glaciers meet and continue to flow downhill together

  • Ground: Material dragged under the base of the glacier and deposited over a wide area on the valley floor

  • Recessional: They show the point of glacial retreat

  • Push: They form if the glacier advances after retreat

Drumlins

• Drumlins are elongated, egg-shaped hills and made of glacial till

• They form beneath the glacier when the glacier meets an obstruction and material is deposited as a ground moraine

• The moraine is then shaped by the moving ice, which follows the direction of the flow of ice

• The largest ones can be over 1km in length, 0.5km wide, and 50m high

• Multiple drumlins are known as swarms or baskets of eggs

• As the material is deposited it builds up to have a round, blunt, and steep front (stoss) end 

• The flow of ice over the top of the drumlin drags the material along and down creating the lee slope

• The lee is gently sloped, elongated, and with a tapered tail 

• The more elongated the drumlin the faster the ice was flowing

• Examples include The Drumlin Field below Cam Fell in the Yorkshire Dales and Conway Valley, North Wales

Lowland Deposition Features

• Lowland ice contact depositional features include:

  • Till plain

  • Lodgement till

  • Ablation till

Till plains or ground moraine

• When the ice mass retreats during the warmer months, till is deposited over a vast area across the valley floor 

• Meltwater will also flow out of the glacier’s snout forming meltwater rivers

• These rivers carry large amounts of glacial till, which will undergo further erosion through attrition to become outwash 

• This finer till is sorted and when the energy of the river reduces, the outwash is deposited in layers further down the valley on the outwash plain

Lodgement till

• Found mainly with slow-moving glaciers which carry more debris sub-glacially

• Lodgement till is subglacial unconsolidated material that is forced, or ‘lodged’ into the bedrock below

Ablation till

• Ablation till includes supra and englacial material deposited as the glacier melts

Former Ice Extent Reconstruction

• Landforms of glacial deposition are useful in understanding the direction and extent of ice cover – striations are deepest at initial point of contact

• It also provides a record of past glacial changes over a longer timeframe than current observational records

• Past temperatures and levels of precipitation can be measured with ice cores, indicating past glacial climates

• Orientation of the landform can indication glacial direction and upland or lowland landforms can be a good indicator of actual ice mass – crag and tail as a macro feature needing a large ice mass to deform around resistant rock rather than flow over as in the meso landform of roches moutonnées 

• Debris analysis can indicate distance travelled, as in the case of the Scandinavian erratics; or the rate of travel as in drumlin swarms, as slow moving glaciers carry more till

• Morainic ridges indicate edges of ice sheets or glacial snouts in instances of glacial retreat