The glacial sole is the bottom-most layer of glacial ice, the point where the glacier touches the ground. It is often referred to as 'the base of the glacier', but that is ambiguous as to whether the 'base' of the glacier is the glacier's sole, or the glacier's terminus.
The glacier's sole is one of the most interesting and least observable parts of a glacier; this is where the most of the glacial erosion takes place. Roches moutonnée, drumlins, Chattermarks, and striations are formed by the sole of the glacier moving across the ground, along with a whole lot of abrasion and plucking. It is also where the ice starts deform and flow plastically, allowing the glacier to flow. Large amounts of rock and rock particles are picked up and entrained, later to be dropped at the glacial terminus.
The headwall of the glacier does is not counted as part of the glacial sole, despite the increased erosion found there.
Events and objects found at the glacial sole are said to be subglacial or englacial.