The story of the Magaliesberg begins underwater. More than two billion years ago, sand and silt settled in layers on the floor of a shallow inland sea, hardening over roughly 300 million years into quartzite, shale, and dolomite atop the ancient Kaapvaal craton. Then the planet heaved. About two billion years ago a vast upwelling of molten rock - the Bushveld Igneous Complex, one of the largest such intrusions on Earth - shouldered into the crust and tilted those flat layers on edge, so the broken rims now face outward and the gentler slopes lean inward. Look closely at the stone today and you can still find the fossilized ripple marks of that long-gone sea, frozen into rock that has outlasted entire oceans.

The land around these mountains may be the oldest home our species has. Just south of the range lie the Sterkfontein Caves, part of the Cradle of Humankind World Heritage Site, where the fossil skull known as Mrs Ples and other ancient hominins were found in deposits reaching back millions of years. Long before any written record, people lived in the shelter of this ridge. The mountains carried many names across the ages - the seTswana speakers knew them simply as Dithaba, the mountains - before the range took the name Magaliesberg in the mid-1800s, after the Tswana leader Kgosi Mogale, whose name carries the sense of sharp and brave. The Magaliesberg has been a human landscape for as long as humans have existed.

The ridge divides the land like a wall. To the north lies the warm, low Bushveld; to the south, the cooler Highveld where winter frost settles in the valleys. That natural barrier made the mountains a prize in war. In 1823 the Zulu commander Mzilikazi broke from Shaka and settled his Matabele nation here, until Voortrekkers and allied Sotho-Tswana chiefdoms drove them north across the Limpopo in 1838. Decades later, during the Second Anglo-Boer War, Boer fighters who knew every hidden kloof used the mountains for guerrilla raids, and the British answered by building stone blockhouses along the ridgeline to choke off their movement. The crumbling ruins of those blockhouses still stand on the heights, weathering slowly into the far older rock beneath them.

For all its antiquity, the Magaliesberg lives very much in the present. In 1923 the Hartbeespoort Dam filled one of its valleys, and the towns of Hartbeespoort and Kosmos grew up around a weekend getaway for the people of Johannesburg and Pretoria; an aerial cableway now lifts visitors to a ridgetop view that spreads the whole landscape below them. The deep ravines carved by ancient erosion have become a climber's paradise, threaded with hundreds of routes up the quartzite cliffs. In June 2015, after years of campaigning, UNESCO declared the range a World Biosphere Reserve - international recognition for a mountain that, as one of its champions said, is a unique treasure: half the age of the Earth, and still teaching us how to live alongside it.