Downstream from Retournemer, the circular movements of the glacier eroded the rock to a depth of 115 ft (35 m). The lake of Longemer formed in this basin. Its first name dates back to an 11th-century manuscript that mentions the locality of the hermit Bilon's cell. The book describes a Mare Longum or long sea, which refers to the elongated shape of the lake. The diversity of its habitats allow the presence of numerous fish. Among these species, the brown trout, the pike, and the river fish are the subject of protection measures.