The islands have been shaped by numerous glaciations during the Quaternary Period, the most recent being the Devensian.[citation needed] As this ended, the central Irish Sea was deglaciated and the English Channel flooded, with sea levels rising to current levels some 4,000 to 5,000 years ago, leaving the British Isles in their current form. Whether or not there was a land bridge between Great Britain and Ireland at this time is somewhat disputed, though there was certainly a single ice sheet covering the entire sea.