A continuous mass of glacier ice with an area over 50 000 km2; Typically an ice sheet is 1–3 km thick. Payne and Sugden (2006), ESPL 15, 7 find that both the vertical amplitude and the spatial distribution of bedrock basins and ridges are important in determining the pattern, rate, and extent of ice-sheet growth. Alley et al. (2005) Science 310, 5747 report that the Greenland and Antarctic ice sheets, if completely melted, could raise sea levels by about 70 m. The resulting freshwater fluxes may affect oceanic circulation. An ice core is a column of ice extracted by drilling into an ice sheet.