Bohusgraniten- Sweden's youngest bedrock.

From Gullmarsfjorden in the south to Idefjorden on the Norwegian border in the north, the mountains along the coast consist of 920 million year old granite. Granite is a so-called eruptive deep rock and this means that it was originally a liquid magma that solidified at a depth of 10-15 km. The erosion during the long time that has passed since this happened and the fact that the earth's crust rose at the same time is the explanation for the granite being found at the earth's surface today. All other bedrock in Sweden is of older date, the oldest, which is found at the very top of northern Sweden is between 2500 and 3000 million years old.
Granite is a silicate-rich rock, which means that it mostly consists of the elements silicon and oxygen. The mineral quartz consists only of silicon and oxygen, feldspar and mica contain in addition to these also some other atomic species such as sodium, potassium and aluminum. Because the crystals were formed at depths where it is not as cold as at the surface, they had time to grow before they were finished. Therefore, granite has a clear grainy structure. The size of the grains varies and if they become really large, the rock type is called Pegmatite instead. The pegmatite most often appears in passages or spots and was called by stonemasons Cat or Cat Stripes.
The color of the granite varies in different shades of pink and gray depending on the type of feldspar and mica included.
There are plenty of cracks in the granite. They have arisen through tensions and that it moves both horizontally and vertically. The horizontal cracks are called bank planes and were used by the stonemasons to make it easier to remove the boulders. The vertical cracks have been eroded by the ice sheets and given rise to the fragmented terrain.
The older rock that the granite penetrated is called gneiss and it is a so-called transformed (metamorphic) rock. Once upon a time, about 1600 million years ago, it was formed by sediment that was carried out into an ancient sea. When the sediment sank into the earth's crust, it warmed up, was subjected to pressure, solidified and had a striped structure. In many places in the granite you can see enclosed older striped pieces of gneiss. The older gneiss remains outside the granite area. Both Kynnefjäll in eastern Bohuslän and Väderöarna and Koster west of us consist of the older gneiss.

In some places in the Bohus granite you can see black passages of a fine-grained rock, diabase. It is of much younger date than the granite. About 275 million years ago, there was a period of seismic turmoil in our area. The bedrock cracked up and out through fissure volcanoes, black magma of a different type than the granite flowed. The solidified on the surface and the black passages are the remains of the long narrow volcanic pipes.
Granite formation is not unique to northern Bohuslän. Similar processes have taken place in many other places both here in Sweden and abroad, but the Bohus granite is the youngest Swedish granite.
About 2 million years ago, a cold period in the history of the earth began, large ice sheets were bitten that weighed down the earth's crust. Inland ice slides and picks up gravel and stones of various kinds that grind and scratch the surface it moves over. This is how the soft rounded shapes have arisen and also the ice grooves that are most clearly visible on the slabs closest to the beach. Many of the foreign stones you find that do not look like granite at all have also been transported with the ice.
The age of the earth is considered by geologists to be about 4,600 million years. When the Bohus granite was formed 920 million years ago, 4/5 of the earth's history had passed. If you take a 4.6 meter (460cm) long rope and let it represent the time of the earth's history, 1cm corresponds to 10 million years. The formation of granite is then 92 cm from the end of the rope which corresponds to the present, fossil shells from marine animals appear 52 cm from the end of the rope, the dinosaurs are on earth between 25 and 6.5 cm into the rope and our own life of max 100 years corresponds to 0.0001mm on rope.