Igneous intrusions are found in the far southern areas of the State. Ages ago North America began to split roughly along the lines of the Ohio and Mississippi Rivers. Igneous rock flowed up into fissures, as a new ocean began to form. Had this continued, Illinois might have resembled the Arabian Peninsula, a broken terrain between two continents. This split, however, failed: the continent reclosed, leaving only a few igneous intrusions, and some oil fields, as evidence that it happened.