1. Basal Slip: This is the primary mode of movement for glaciers resting on a bed of soft, deformable material like clay or till. The glacier slides along the base due to the pressure of its weight and the lubrication provided by meltwater at the base. This process is similar to a sled sliding on a snow-covered hill.

2. Internal Deformation: This is the dominant mode of movement for glaciers resting on a rigid bedrock, where basal slip is limited. The glacier's own weight causes the ice to deform internally, behaving like a very viscous fluid. The ice crystals within the glacier slide past each other, creating a slow, plastic flow. This process is akin to a very slow-moving river.

It's important to note that glaciers often exhibit a combination of both basal slip and internal deformation, depending on the conditions of their base and the surrounding environment.