[4] The widest definition of the range includes the Garraun Complex to the north as well as several isolated peaks to the west, and is designated a 16,163-hectare (39,940-acre) Special Area of Conservation.

[17] Movements in the Earth's crust, and the closure of the Iapetus Ocean, transformed these sediments into crystalline schists that lie underneath the base of the mountain range, which local erosion and uplift then brought to the surface.

[17] In contrast, the mountains to the north of the core Twelve Bens massif, the Garraun Complex, have a different type of geology, that is composed of gneiss and different forms of sandstones and mudstones.

[5] Scattered throughout the range are parts of gabbro (Doughruagh and Currywongaun), mica schist (Muckanaght), and outcrops of marble (south of Kylemore Lough).

[5] The final Ice Age, circa 10,000 years ago, also sculpted the landscape leaving behind deposits of sand and gravel; there are widespread boulder-clay and erratic boulders across the range.

[5] The range is popular with hill walkers, fell runners, and rock climbers,[18] The Irish Times outdoors correspondent, John G. Dwyer, said of the Twelve Bens, "These are true kickass mountains, with criminally stunning views [..]".

[19] The 16-kilometre (9.9 mi) 8–9 hour Glencoaghan Horseshoe[d] is noted as providing some of the "most exhilarating mountaineering in Ireland",[23] and is called "a true classic" by guidebook authors.

[e] The Twelve Bens have a number of rock climbing locations, the most notable of which is in the Gleann Eighneach valley at the eastern spur of Benncorr (from Binn an tSaighdiúra to Bencorrbeg; also called "Carrot Ridge").