Windermere is the largest natural lake in England. More than 11 miles in length, and almost 1 mile at its widest, 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. Forming part of the border between the historic counties of Lancashire and Westmorland, Windermere is today within the administrative county of Cumbria and the Lake District National Park.