The first of these, plastic flow, involves movement within the ice.
Ice behaves as a brittle solid until the pressure upon it is equivalent to
the weight of about 50 meters (165 feet) of ice.
Once that load is surpassed,
ice behaves as a plastic material and flow begins.
A second and often equally important mechanism
of glacial movement consists of
the entire ice mass slipping along the ground.
The lowest portions of most glaciers are thought to move by this sliding process.