The movement of the African plate, together with the contiguous Arabian plate during the Tertiary created compression, which resulted not only in the major folds of the mountain ranges of the north of the region but, more important as far as the contemporary economies of the region are concerned, they also created the minor folds characteristic of parts of Iraq and Iran, as well as onshore and offshore tracts of the Persian/Arabian Gulf. These gentle folds trapped countless reservoirs of crude oil and natural gas, which have proved to be the richest deposits in the world.