Giant's Causeway cases the horizontal fracture has resulted in a bottom face that is convex while the upper face of the lower segment is concave, producing what are called "ball and socket" joints. The size of the columns is primarily determined by the speed at which lava from a volcanic eruption cools. The extensive fracture network produced the distinctive columns seen today. The basalts were originally part of a great volcanic plateau called the Thulean Plateau which formed during the Paleocene. According to legend, the columns are the remains of a causeway built by a giant. The story goes that the Irish
Geological Exploration of the Fortieth Parallel Geological Exploration of the Fortieth Parallel The Geological Exploration of the Fortieth Parallel was a geological survey made by order of the Secretary of War according to acts of Congress of March 2, 1867, and March 3, 1869, under the direction of Brig. and Bvt. Major General A. A. Humphreys, Chief of Engineers, by Clarence King, U. S. geologist. More commonly known as the Fortieth Parallel Survey, the survey conducted field work from 1867 to 1872, exploring the area along the fortieth parallel north from northeastern California, through Nevada, to eastern Wyoming. The results of the survey were published in