The islands of Saint Bartholomew and Saint Martin consist of andesitic tuffs and tuff-breccias of Middle and Late Eocene (?) age, respectively, which have been intruded by hypabyssal basalt, andesite, and quartz diorite of a slightly later age. No older “basement rocks” are present as previously supposed. During the late Eocene and early Oligocene, these islands are believed to have been the sites of active volcanoes whose centers of activity shifted from east to west. The tuff series were tilted and faulted as a result of the volcanic activity and on St. Martin they were extensively metamorphosed.
By the end of the Oligocene, the area had been eroded to the roof level of the intrusive rocks and the Oligocene-Miocene limestone and marls were deposited unconformably on the tuffs. On Anguilla, limestone of a similar age covers the entire island except for two small outcrops of tilted tuffs and basalt.
At some later date, the limestones were gently folded, and, during the Pliocene and Pleistocene, the islands were probably connected to form one large island. The area is now submerged to form the Anguilla bank with the mountainous portions being the present islands.