The flow is usually slow, amounting to a few centimetres per day for large ice sheets, though certain active glaciers can move as fast as several metres per day. However, sometimes certain glaciers may go through episodes of very rapid movement. We say that the glacier is experiencing a surge. A surging glacier may travel by more than 60m per day for several months (Strahler and Strahler, 1994). Most glaciers, though, do not experience surging.