Geologically, the Brenta Group is very different from the granite-formed neighboring mountain groups like Ortles and Adamello/Presanella. The main peaks are formed of hard compact dolomite, while the peripheral subgroups often are made up of more calcareous dolomite or limestone. The hard dolomite (dolomia principale) was originally formed during the Mesozoic era, under the surface of the shallow Tethys Ocean, some 200 million years ago. The hard compact dolomite with a high magnesium content was formed during the Upper Triassic period. The softer, more calcareous material was deposited later, in the late Triassic and early Jurassic period. The difference is clearly noticeable for the climber who gets a much more compact and reliable rock on the peaks of the central part of the Brenta Group than in the peripheral subgroups. Eventually, these layers were pushed upwards by the tectonic activity that led to the formation of the Alps during the Eocene, starting some 66 million years ago and leading to the formation of folds and thrust faults. Subsequent erosion carved out the dolomitic landscape with its steep vertical pinnacles, as we know it now.