Originally there was an isthmus across the Strait of Dover. During an earlier glacial maximum, the exit from the North Sea was blocked to the north by an ice dam, and the water flowing out of rivers backed up into a vast lake on the bed of what is now the North Sea. A gently upfolding chalk ridge linked the Weald of Kent and Artois, perhaps some 30 metres (100 feet) higher than the current sea level.