The geology of Shetland is complex with numerous faults and folds. These islands are Scotland's most northerly area of Caledonian orogenic rocks 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 are remnants of the Iapetus Ocean floor. Much of Shetland's economy depends on oil and gas production from fields in the surrounding seas.