Modelling ice sheet behavior in the context of climatic changes depends on initial and boundary conditions which can be better defined by studying the composition of basal ice. This study deals with basal ice reached by deep drilling at Summit in Central Greenland (GRIP core). The isotopic composition of this ice indicates that ice formed at the ground surface in the absence of the ice sheet largely contributed to its formation. The basal silty ice is a remnant of a growing stage of the ice sheet, possibly the original build up.