=Limits.=—The ice of a glacier is always moving forward (neglecting temporary halts), but the _end_ of a glacier may be retreating, advancing or remaining stationary, according as the rate of wastage is greater, less, or just equal to the forward movement of the ice. The position of the lower end of the glacier is therefore determined by the ratio of movement to wastage. Its upper end is generally ill-defined. In a superficial sense, it is the point where the ice emerges from the snow-field; but the lower limit of the snow-field is often ill-defined, and in any case is not the true upper limit of the glacier, since there must be movement from the granular mass of ice beneath the snow to make up for the waste below, and the moving ice beneath the snow-field which feeds the tongue of ice in the valley is just as really a part of the glacier as the more consolidated portion in the valley below. If a definite upper limit for an alpine glacier is to be named, it should probably be the _Bergschrund_, a gaping crevasse, or series of crevasses which sometimes open near the precipitous slope of the peak or cliff where the snow-field lies. The _Bergschrund_ is formed by the moving of the lower part of the snow-field away from the portion above. The lower end of a glacier is usually free from snow and névé in summer, but, traced toward its source, it first becomes covered with névé, then with snow, and finally merges into the snow-field without having ceased to be a glacier. The term glacier is, however, commonly used to mean merely the more solid portion outside (below) the névé. =Movement.=—The fact of glacier movement is established in various ways, the most obvious being by the advance of its lower end. Such advance is too slow to be seen from day to day, and is only detected when the lower end of the glacier overrides or overturns objects in front of it, or moves out over ground previously unoccupied. But even when the end of a glacier is not advancing, the movement of the ice may be established by means of stakes or other marks set on the surface. If the positions of these marks relative to fixed points on the sides of the valley be determined, they are found after a time to have moved down the valley. Rows of stakes or lines of stones set across a glacier in the upper, middle and lower portions have revealed many facts concerning the movement of the ice.