When scientists first studied glaciers in the early nineteenth century, the complexities of glacier flow were unknown. Combining mountaineering with science, people such as James Forbes and John Tyndall set about making measurements of the flow of valley glaciers such as the Mer de Glace in France and the Unteraargletscher in Switzerland. Some of the fundamental aspects of glacier flow were determined at this time. Today we recognize that there are a wide range of features that indicate glacier flow. The formation of crevasses and other structures, the displacement of rocks on the surface and the occasional cracking and creaking sounds within the ice are all symptoms of this. The eroded rocks and deposits that are left behind after the glacier has receded also indicate how glaciers move.
Rates of movement of flowing glaciers are extremely variable. Some small glaciers and ice caps may flow only a few metres a year. On the other hand, the fastest part of an average-sized valley glacier flowstypically between 50 and 400 metres a year, even several kilometres if they end in the sea. Similarly, large ice streams that drain the Antarctic and Greenland ice sheets flow steadily at rates of several kilometres a year.
A small percentage of the world's glaciers flow in a rather unpredictable manner; they remain relatively inactive for many years, but may accelerate suddenly, increasing many hundredfold in speed.