 The geology of Shetland is quite different . It is extremely complex , with numerous faults and fold axes . These islands are the northern outpost of the Caledonian orogeny and there are outcrops of Lewisian , <unk> and Moine metamorphic rocks with similar histories to their equivalents on the Scottish mainland . There are also small Old Red Sandstone deposits and granite intrusions . The most distinctive feature is the ultrabasic ophiolite , peridotite and gabbro on Unst and Fetlar , which are remnants of the Iapetus Ocean floor . Much of Shetland 's economy depends on the oil @-@ bearing sediments in the surrounding seas . 
