Valley glaciers have been called "rivers of ice." Images like this make it easy to visualize that the ice is moving downhill under the force of gravity. Though glacial ice is solid, these thick masses of compressed snow and ice behave like a very viscous fluid, moving slowly downhill.
If you were to watch a valley glacier
for several hours, or if you just were to look at it
every day for several weeks, it would be hard to tell
that it was moving. However, flow rates for glaciers
are very commonly measured and reported.
1. How could you measure the speed at which a glacier flows?