Ice sheets have a distinctive surface topography, which includes features such as 
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nunataks, piedmont glaciers, ice lobes, and ice shelves. The land beneath the ice 
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is often depressed to below sea level by the weight of the ice. In this case the 
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icecap covering the subducted areas is dome-shaped from the marginal areas to 
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the interior. The movement of the ice is outward from the raised center, and the 
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rate of flow increases as the ice flows down to the sea and through mountain 
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valleys. The ice moves either as lobes through valleys or as part of the ice shelf 
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into the sea where it melts or calves into icebergs. The friction of the moving ice 
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on the underlying rock or the weight of accumulated ice can lower the freezing 
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point of the bottom layers of ice and cause them to melt.