CORE UNIT: PATTERNS AND PROCESSES IN THE PHYSICAL ENVIRONMENT
Upland Accumulation Crevasse zone
Erosion
Erosion
Ablation Lowland Deposition
Fig. 11 Diagram and photograph to show accumulation, ablation and periglacial zones of a glacier in Alaska. The broken red line in the photograph shows where snow build-up (accumulation) gives way to melting (ablation). Above the red line fresh snow is whiter. Below the red line the melting ice is greyer in appearance.
Glacial erosion Plucking
The weight of the ice puts incredible pressure on the base and sides of a valley. This causes the base and sides of the ice to melt, creating a thin film of meltwater for the ice to slide over. This meltwater seeps into cracks and crevices in the rock. When it refreezes it expands, crumbling the rock and sticking to it. Later when the icemoves it drags or plucks the rock fragments away. Pluckingworks in the sameway aswhen you take an ice cube tray from the freezer and your fingers stick to it.
Abrasion
The plucked rock fragments embedded in the glacier act like sandpaper as the ice carries them. They scrape and gouge the landscape as the ice moves, creating deep scratches called striations or striae. Striae are very common on Irish glaciated landscapes and may help to indicate the direction of ice movement.
Fig. 12 Glacial striae, County Kerry Glacial transport
The rock fragments plucked and abraded by the ice are carried along with it. This material is called moraine. Moraine carried along while stuck to the bottom of the ice is
called groundmoraine.Moraine that finds its way into the ice through crevasses is called englacial moraine. Rocks from the valley walls that are broken up by frost
shattering (freeze-thaw action) fall onto the sides of the glacier to accumulate as lateral moraine. You will learn more about moraines later in this chapter when studying depositional landscapes created by ice.
Fig. 13 Moraine deposited by ice in the French Alps 146