About four-fifths of the way to the summit, at an elevation of 1,400 meters, is the Ontahara Wetland. It is a treasure trove of alpine vegetation, with over 130 varieties including the chocolate lily, the Geum pentapetalum, the daylily, the Miyama edelweiss, both the Asian and Eastern skunk cabbages, and others. In particular, one can see around the Iroha Yonjuhachi-numa pond an impressive display of alpine plants.
In the center of the Ontahara Wetland is Ontahara Shrine. The shrine serves as the middle temple of Gassan Mountain, and handles prayers and talismans. Ontahara Koya serves as the resting place for pilgrims in Ontahara. Many pilgrims heading for Gassan Shrine stop here to eat and spend the night.
Gassan Shrine, which lies at the summit of Gassan Mountian, is the only kanhei taisha shrine established by the engi system in the Tohoku Region. It is ranked as a juni-i shrine and has been revered since ancient times, first by the Imperial Court and then by the lower classes. In Yamagata City there is a stone marker inscribed with the date 1365 that speaks of over 100 people in the same village who shared this faith. The deity Tsukiyomi-no-Mikoto is enshrined here. Tsukiyomi-no-Mikoto is the deity of night, the sea, the spirit, and the world after death (reincarnation and rebirth). Represented as the moon, this deity grants miracles related to peace on the earth, the tranquility of the nation, the development of industry and commerce, bountiful harvests, and plentiful fishing. Gassan Mountain is also revered as a place for the respite of ancestral spirits. During the time the mountain is open the Gassan Saito Festival is held at the summit to honor the ancestors and the spirits that live within rice plants.
The view from the summit affords a magnificent panoramic view in all four directions.