It was 20 millions years ago that Britain and Ireland ended up where they are now. Almost 300 million years before that, the super continent Gondwanaland had began to separate in a north-south movement. Then 100 million years ago this shifted a an east-west separation, and by 40 million years ago the Atlantic Ocean was formed – and the continents as they are now were more or less in place 20 million years ago.
The geology of Ireland and Britain reflects the various states of formation – everything from ice ages to tropical climates, and volcanic activities to changes in sea level. The end of the lce Age about 10,000 years ago was not even the final dramatic act. During the Ice Age the central part of the Irish Sea was probably a long freshwater lake. As the ice melted 10,000 years ago the lake became connected to the sea, becoming slightly salted (brackish) and then fully saline.
The Irish Sea is a shallow sea-floor region that lay near the former ice sheet that occurred over Ireland and Britain until about 13,000 years ago.
Caught between a Land-bridge and a Glacier.
There are estimates that the ice thickness over the Irish Sea was at a maximum of about 600 m. It is likely that a land-bridge between Britain and Ireland existed across the Celtic Sea from about 18,000 to 14,000 years BC across the Celtic Sea from Cornwall to southern Ireland. No land connection is believed to have happened further north, but the Isle of Man is believed to have been connected to northwest England briefly from about 14,000 to 11,000 years BC. Caught between this land-bridge, and the ice sheet in the north of Ireland, the Irish Sea was filled forming a vast freshwater lake.
Despite it being commonly believed, there was no land connection between northern Ireland and Scotland. Even the land connection across the Irish Sea might have been a weak one of between five and one metre high (between 18000 and 14000 BC). This exposed sea floor would not have been a fertile place for any flora or fauna attempting to cross to Ireland. The fossil record shows limited faunal contact between Ireland and Britain in Late Pleistocene and Holocene time.
Plant life returned to reclaim the rocky wilderness that Britain and Ireland had become. First rugged grasses coated the land and, around 13,000 years ago, the first trees (hardy Junipers) began to grow. Many animals, including the Giant Deer, crossed into Ireland across the land bridge.
The low-lying lands around Ireland, Britain and Europe were slowly flooded as the sea rose at a maximum rate of around 6mm/year. The land-bridge between Ireland and Britain was overwhelmed by the sea 12,000 years ago, flooding the fresh water Irish Sea with salt water.
Limits of the Irish Sea
The International Hydrographic Organization now defines the limits of the Irish Sea (with St George’s Channel) as follows:
On the North. The Southern limit of the Scottish Seas [or Inner Seas off the West Coast of Scotland, defined as “a line joining the South extreme of the Mull of Galloway (54°38’N) in Scotland and Ballyquintin Point (54°20’N) in Ireland”].
On the South. A line joining St. David’s Head (51°54′N 5°19′W) to Carnsore Point (52°10′N 6°22′W).
Late Pleistocene and Holocene sea-level change – K. Lambeck, P. Johnston, C. Smither, K. Fleming and Y. Yokoyama – http://rses.anu.edu.au/geodynamics/AnnRep/95/AR-Geod95.html
Ireland in the Ice Age – http://www.wesleyjohnston.com/users/ireland/past/pre_norman_history/iceage.html
A collection of findings from Archaeology, Geology and other scientific endeavor – http://www.rootsweb.ancestry.com/~irlkik/ihm/ancient.htm