The Grebensteine nature reserve, right next to the ski jump in Willingen, is an impressive ravine forest with primeval forest-like vegetation. Due to the numerous large boulders, the area is also known as a block pile forest and covers an area of 15.70 hectares. The rocks are made of gray clay slate and are permeated by numerous springs, which makes for a unique flora.

Summer lindens, wych elms and sycamores grow in this area. The high humidity and low temperatures create a montane climate in which alpine plants also thrive. Particularly noteworthy are the large populations of perennial silverleaf (moon violet). Other plant species such as daphne, alpine red currant, marsh fritillary, yellow shield fern and alpine witchweed find ideal growing conditions here.

The Grebensteine nature reserve is also a paradise for butterfly lovers. A total of 42 butterfly species, which are adapted to the cool, damp, montane climate, have been recorded here.

The W1 hiking trail leads along the Grebensteine and offers hikers the opportunity to experience the beauty of this special biotope up close. However, access to the nature reserve itself is not permitted in order to protect the sensitive natural environment.