Cwm Idwal was designated as a National Nature Reserve by the Nature Reserve Board in 1954. The Reserve stretches from 1,200 to 3,000 feet above sea level (365 to 914 meters). The valley is of special interest to geologists and geographers. Around 10,000 years ago, after the thawing of the last Ice Age, four lines of hills formed at the bottom of the valley.
Llyn Idwal and its surroundings sustains various plants that have specific qualities of high altitude lakes and Twll Du is rich in Arctic-Alpine plants that are usually found in the cold countries of northern Europe and Scandinavia. The reserve continues to be of high importance to education and scientific research, as well as being a hub for visitors, hikers and climbers.