The geology of Shetland is complex with numerous faults and fold axes. These islands are the northern outpost of the Caledonian orogeny and there are outcrops of Lewisian, Dalradian and Moine metamorphic rocks with similar histories to their equivalents on the Scottish mainland. Similarly, there are also Old Red Sandstone deposits and granite intrusions. The most distinctive feature is the ultrabasic ophiolite, peridotite and gabbro on Unst and Fetlar, which is a remnant of the Iapetus Ocean floor. Much of the island's economy depends on the oil-bearing sediments in the surrounding seas. In the post-glacial epoch, , the islands experienced a tsunami up to 20 metres high caused by the Storegga Slides, an immense underwater landslip off the coast of Norway.