Windermere is the largest natural lake in England. It is a ribbon lake formed in a glacial trough after the retreat of ice at the start of the current interglacial period. It has been one of the country's most popular places for holidays and summer homes since the arrival of the Kendal and Windermere Railway's branch line in 1847. Historically forming part of the border between Lancashire and Westmorland, it is now within the county of Cumbria and the Lake District National Park.