 The Wyville Thomson Ridge, and the smaller but similar Ymir Ridge, form the northern boundary to the Rockall Basin, a mainly Mesozoic rift structure.The current form of the ridge is an anticline with up to 2 km of amplitude, formed by a period of shortening during the Eocene to Miocene period.This fold is interpreted to have formed by the reactivation of a pre-existing fault, and is, therefore, classified as an inversion structure.