During the next 200 million years, the rocks of the Aar Massif remained virtually unchanged. During the alpine mountain-building phase around 40 million years ago, the Massif was subjected to strong loading. As a result of transport of the alpine nappes towards the north, the Aar Massif subsided and was overlain. At the time of the maximum overburden (around 12 km), the rocks were exposed to high temperatures (around 450°C) and pressures (around 300 MPa). This phase saw the formation of schistosity and shear zones. In the subsequent phase of uplift around 16 million years ago, tension joints or fissures formed in the rock; these are partly filled with beautiful crystals. Uplift is still ongoing today at a rate of around 0.5 to 0.8 millimetres per year.
Geological-tectonic profile along the Grimsel pass road