Windermere is the largest natural lake in England. More than 11 miles (18 km) in length, and almost 1 mile (1.5 km) 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.

Windermere is long and narrow, like many other ribbon lakes, and lies in a steep-sided pre-glacial river valley that has become deepened by successive glaciations. 

The current lake was formed after the Last Glacial Maximum during the retreat of the British and Irish Ice Sheet some time between 17,000 and 14,700 years ago, just before the start of the Windermere Interstadial.

- Wikipedia