What is "maridalen"

Maridalen is a valley situated just north of Oslo, Norway, just above the suburb of Kjelsås. Most of the valley is made up of a lake known as Maridalsvannet, that serves as the primary source of drinking water for 90% of Oslo's population. The valley is a popular recreational area with many hiking trails and cycle tracks for summer usage, and groomed cross country trails in winter. Both the lake and valley are protected.