The best preserved of these is found in Espevik on Tysnes. When these dramatic events unfolded, what is now the earth's surface lay several kilometres underground. Today, we see the remains of the magma that filled the cracks in the earth's crust. The magma followed the many steep faults along the coast, faults that created valleys, straits and fjords trending in a north-northwesterly direction. This fracture system must have stretched quite far down, to great depths. The magma came from over 50 kilometres depth (the mantle) and has intruded right through the crust on its way to the surface. The magma that was left in the fractures can be found as diabase sills, that is, fine-grained alkaline magmatic intrusions.
One finds nearly a hundred such sills, from Sund in the north to Sveio in the south. At the head of the bay in Espevik one can see many of these dark sills. Here, they have also been dated. By studying the isotopes in the element argon, it has been discovered that they are about 220 million years old (from the Triassic Period). In other places (i.e. on Sotra), the dating reveals intrusions also in Permian times, about 250 million years ago. This is quite far back in time, but the sills are nonetheless the youngest rocktypes on dry land in Hordaland. Only in the undersea Bjorøy tunnel is the bedrock younger.
