Greenland Ice Sheet stratigraphy. Cross-section showing combined results of studies of the age of the Greenland Ice Sheet, a mass of ice with an area of 1.7 million square kilometres, reaching a thickness of over 3 kilometres. From top, the layers are from the Holocene period (green), formed during the past 11.7 thousand years; during the last ice age (blue), from 11.7 to 115 thousand years ago; and from the Eemian period (red), more than 115 thousand years old. Regions of unknown age are shown in grey. This research was published by MacGregor et al in 2015. The research included data collected by satellite, ice cores, and aircraft flights.