Near the Stieregg and just south of Bäregg passes a juncture of two differently old geological formations of the Swiss Alps. It separates the older, between 1.2-2 billion years old Aar massif (crystalline basement) in the south from its much younger, about 120-160 million years old sedimentary cover, which consists of limestones of the Upper Jurassic and Lower Cretaceous, and forms the northern edge of the Aare massif (Autochthon of the Helvetic shield). The limestones are mighty sediment packages that were deposited on the northern edge of a huge ocean basin between the African and the European continent, the Tethys. During the late phase of the Alpine folding, which began 65 million years ago, these limestone sediments were heavily stressed by the compression of the two continental plates, but remained more or less at the same place (autochthonous). Splinters of the crystalline basement penetrated here and there into the highly compressed, folded and fractured limestone beds. Partially the crystalline basement pushed even over the limestone layers as it was the case for the Jungfrau and the Mönch. The banded and flatter limestone layers are clearly visible in the glacier gorge on the eastern edge of the Eiger. The sharp-edged grounds of dark gray limestones contrast with the soft and rounded shapes of the crystalline basement. This consists largely of gneisses, containing mainly quartz, feldspars and mica. The gneiss received its leafy structure during the formation of the Alps. The original rocks were highly transformed by high pressure and temperatures. In this context they are called metamorphic rocks.