Glaciers have a variable impact on rock beds. Key factors include ice thickness - this determines the effective pressure exerted on the blocks which are the agents of erosion at the glacier base. Ice velocity is important, as this controls the rate that rate at which abrading debris crosses the rock floor, and links to other factors like gradient and snow fall. The temperature of ice at the base of the glacier is a fundamental control. If the ice remains below its pressure melting point then it is frozen to its bed and movement can only take place within the glacier. No abrasion can occur and plucking is precluded by the lack of meltwater to allow freezing on of debris. Where ice can melt - and this will occur well below 0ºC where high pressures are generated beneath thick ice - then sliding can take place at the glacier base and both abrasion and plucking can be effective.
The pattern of basal melting may vary between the inner and outer parts of an ice sheet, reflecting differences in air temperatures, snow accumulation rates, gradients, topography and other factors. In the ice shed zone, rates of ice flow are often low and so this trends to be a zone of limited erosion. At the equilibrium line, where ice build up by snow fall and loss by melting are equal, ice discharge is at a maximum and this is a zone of maximum erosion. Towards the terminus of a glacier, the ice is thinning and slowing down and this is the zone of deposition. When glaciers change in size and dynamics as they build up and decay, the zones of glacial erosion will shift through time.
This simple zonation neglects the importance of ice streams in glacier flow. Many ice sheets have zones of rapid flow where ice discharge rates can be 10 or more times faster than in intervening areas. Typically, ice sheets stream along valleys, enhancing erosion and deepening valleys.
Zones of glacial erosion
Shetland is a narrow archipelago which falls almost entirely with a zone of effective glacial erosion. The depositional zone for the last ice sheet lies largely offshore, although the remnants of older Pleistocene deposits at Fugla Ness and Sel Ayre and the extensive weathered rock on Uyea indicate the preservation of delicate materials at protected, lee sites on the western fringes of Mainland.
In the western parts of Shetland ice-moulded scenery is widespread, especially on rock types that generate large blocks. Striae and roches moutonnées indicate the movement of ice to the west and northwest.
On the east coast the indicators of directions of ice movement are reversed. Here the rock basins and ice-moulded crags attest to ice movement towards the east and northeast.
Along the ridges and valleys of central Mainland lies the ice divide and here the topography is less knobbly. The smoother slopes reflect the reduced erosion in the ice shed zone but there is also a lithological control, with the fissile mica schists producing fewer crags. This is also a zone of few striae in which shattered and weathered rocks have survived the passage of ice.
Is there anywhere on Shetland that was not covered by warm-based, eroding ice? Perhaps the summit of Ronas Hill - ice moulded terrain is limited to ground below the summit of Collafirth Hill and the terrain above this carries glacially-disturbed granite regolith rather then deeply eroded bedrock. A small patch of bedrock W of the summit carries weathering pits up to 15 cm deep that may relate to weathering in the last interglacial. Other frozen bed patches may exist but a lot of peat needs to be dug before they can be recognised!