- The shape of the tunnel is rectangular. With some imagination, one can say that it resembles a large window!
- The rock is gneiss. On the walls and ceiling you see a layered structure in the gneiss and fracturing which either goes along or across the layering in the rock. Structure and fractures often follows the same direction as the roof, walls and floor of the tunnel. When water penetrates between the cracks and freezes, it expands and splits the rock. This is called frost-weathering (see figure below), and has contributed to the formation of the tunnel, its shape, and many relatively flat and angular stones in the bottom of the tunnel. So structures and weathering explains why the tunnel is rectangular and the direction of the structures has clearly contributed to the formation of a tunnel.
- Why it was formed a gigantic tunnel here is, however, uncertain. We know that Torghatten was glaciated in re-occurring ice ages. And we know that sea level has been much higher than today when the ice-sheet pressured down the land. In fact, if you had been standing here c. 13 000 years ago, you would have been wet and cold by waves. Most likely multitude processes have contributed to the formation of the tunnel during many ice ages and ice-free periods. Glacier, melt-water from the glacier, waves, and frost-weathering all contributed to the formation. The glacier and high-pressure melt-water probably served as a giant drill which honed and expanded the tunnel. If you look carefully, you can see rounded rock walls, indicating smoothing by both glacier and melt water. And the first 2-3,000 years after the glacier disappeared, powerful storm waves from the Atlantic sea probably knocked in the openings on both sides, tearing off slabs of rocks.
The figure above shows the processes involved in frost-weathering of rocks.