Glaciers are huge and slow moving rivers of ice which exist in various parts of the world: Alaska, the Rockies, the Alps, Spitsbergen, China, for example. They drain areas in which snow accumulates, much as rivers drain catchment areas where rain falls. Glaciers also flow in the same basic way that rivers do. Although glacier ice is solid, it can deform by the slow creep of dislocations within the lattice of ice crystals which form the fabric of the ice. Thus, glacier ice effectively behaves like a viscous material, with, however, a very large viscosity: a typical value of ice viscosity is 1 bar year (in the metre-bar-year system of units!). Since 1 bar = 105 Pa, 1 year ≈ 3 × 107 s, this is a viscosity of some 1012 Pa s, about 1015 times that of water. As a consequence of their enormous viscosity, glaciers move slowly - a typical velocity would be in the range 10–100 m y-1 (metres per year), certainly measurable but hardly dramatic. More awesome are the dimensions of glaciers. Depths of hundreds of metres are typical, widths of kilometres, lengths of tens of kilometres. Thus glaciers can have an important effect on the human environment in their vicinity. They are also indirect monitors of climate; for example, many lithographs of Swiss glaciers show that they have been receding since the nineteenth century, a phenomena thought to be due to the termination of the ‘little ice age’ in the middle ages.