The surface formed at the time of minimum Annual mass at each point on the glacier, marking (in the stratigraphic system) the end of one mass-balance year and the start of the next. In general the summer surface is diachronous. For example, when the higher reaches of a glacier start to gain mass, the lower parts may still be ablating. The summer surface is the surface on which the first snow of the new balance year falls. It is easily detectable when it consists of glacier ice, which now includes superimposed ice added during the previous balance year. In the firn area it is recognizable as a well-marked crust, that is, a thin, relatively strong layer with a density near that of ice, and sometimes also (or instead) as a layer of depth hoar at the base of the current year's accumulation. The crust typically originates by recrystallization of the surface snow in late summer to form glaze. It may also be marked by an accumulation of sediment or wind-blown dust. It can be difficult to detect when melting and snowfall alternate during the transition between the ablation season and the accumulation season. In some mass-balance programmes the summer surface is 'labelled' in the vicinity of stakes with a distinctive material, such as sawdust, during a visit late in the ablation season.