For a fully detailed technical overview of the Grimsel geology please view : Geology of the Grimsel Test Site (GTS)

Around 300 million years ago, granitic melts solidified at a depth of around 10 - 13 km. The volume of the rock decreased due to cooling and deep fracture systems formed. Residual magma rose through these to form dyke rocks (lamprophyres and aplites).

The rocks of the Aar Massif remained virtually undisturbed for more than 200 million years. Extensive deformation of the rock body then began during the course of the alpine orogeny, around 40 million years ago. The Aar Massif subsided and was overlain by the alpine nappes moving towards the north.

At the time of maximum overburden (approximately 12 km), the rock was exposed to high temperatures (around 450°C) and pressures (around 300 MPa). The main schistosity and shear zones were formed during this period.

The crystalline rock in the Grimsel area has long been thought of as a "Massif", a large block of crystalline basement pushed up through the overlying sediments. However, recent observations in the new Lötschberg railway tunnel indicate that it is, in fact, a massive thrust sheet, overlying sediments in some areas.

In the uplift phase - still continuing with a rate of around 0.5 to 0.8 mm per year today - the tension joints with their beautiful crystals (see The Crystal Cave) were formed around 16 million years ago.