Sea level has risen 120 metres over the last 18 000 years, flooding what we call the continental shelf. Ireland became insular around 13 000 years ago: The French and British prehistoric man had only a river to cross to meet. This river, the paleo-channel, now extinct, drained the Seine, the Somme, British rivers and a glacial lake.
A few explanations
Earth’s rotation around the sun causes insolation variations. This is of course true for the night-to-day transition but also applies to much longer periods of up to several hundred thousand years.
The first half of the Quaternary is marked by alternating hot and cold periods approximately every 40 000 years and the second half by cycles of around 100 000 years. Variations in insolation provoke a number of changes such as the climate with consequently variable quantities of water trapped in ice caps.
- During cold periods, sea levels drop and ice caps can become several kilometres deep. Continental crusts loaded with ice slowly sink.
- As the climate warms, sea level increases through thermal expansion of seawater, water from melting ice caps and local uplift of the continental crust in response to ice sheet discharge. The latter factor is delayed in time by around ten thousand years.
Graphic animation from a model based on:
- Sea level variation graph (Fairbanks, 1989 ; Figure 1) ;
- Glacio-hydro-isostatic model (Lambeck, 1995) taking account of the continental crust uplift (Figure 2) ;
- Ice-cap limits on the British Isles (British Geological Survey) and in the Alps (Furrer et al., 1987) ;
- A morphological summary (Bourillet, 1999) including:
- IFREMER data from the EEZ programme (Zone Economique Exclusive), and SEDIMANCHE 1, SEDIMANCHE 2 and SEDIFAN expeditions;
- Elevation data from Smith and Sandwell, maps from ISTPM, CNEXO, BGRM and SHOM (Figure 3) ;