(Image of Malaspina Glacier. NASA/JPL)
Solid ice is a type of rock. Rocks generally do not have fluid properties, but if you put enough pressure on then, they can do weird things. Glaciers
have a lot of ice, and ice is heavy. Ice weighs around 920 kg/m^3, and glaciers (or ice sheets) can get up to several km thick. F=ma, right? Well, that
is a lot of m, and when under such pressure ice starts deform. That deformation is a non-newtonian flow, similar to substances like motor oil, ketchup
and,particularly, NOT water. The above image is a (rendering) of a glacier based on satalite imagery, but it is easy to see the similarities between the
glacier and a river. We can actually simulate glacier ice, to an extent, in a lab using a substance called "flubber."
(Flubber "glacier." Image from Leigh Stearns)
The similarities are not a coincidence. There is a lot more to glacier movement than the flow of the ice, but it is a major factor, and understanding
the interactions between the ice and the surrounding systems is an important part of understanding glaciers, and ice, as a whole.