Sub-stage· The columns of time

Last Glacial Maximum

26.5 ka → 19 ka

The Last Glacial Maximum is the seven and a half millennia, from 26,500 to 19,000 years ago, when the ice sheets were at their largest.

Interval of seven thousand five hundred years, from 26,500 to 19,000 years before present, corresponding to the maximum extension of the ice sheets. Sea level stands about 130 metres below the present level.

What the evidence shows

Clark and colleagues defined the interval by seeking the period during which the various ice sheets were simultaneously at or near their maximum extent. Lambeck and colleagues established its marine consequence by inverting a thousand observations of ancient shorelines. Inversion is here the decisive method: each ancient beach supplies sea level at one place and one date, but combined with the vertical movement of the ground, so that these measurements cannot be averaged. A mathematical model of the Earth, which calculates how a given history of ice sheets and seas raises or lowers the ground and the shore at each site, is therefore used the other way round: it looks for the single history that best reproduces all the observations. The result is a history of global sea level: it falls to 134 metres below the present level between 29,000 and 21,000 years ago, and is still very low when the main phase of deglaciation, the melting of the ice sheets, begins around 16,500 years ago.

Bounds and conventions

A maximum is more easily defined than dated. The various ice sheets peaked at slightly different dates, and the interval retained corresponds to the window during which nearly all of them were close to their maximum, a stricter definition than those used elsewhere and consequently shorter.

The maximum extent of the ice sheets, from 26,500 to 19,000 years (Clark et al. 2009).