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
Fort Rock lack the heavy metal signature residues of space objects. Crack in the Ground and Fossil Lake are two more nearby Ice Age geological features. South Ice Cave is a lava tube. Derrick Cave is estimated to be long, perhaps Oregon's third-longest lava tube cave. Devil's Garden Lava Field and East Lava Field are other geological oddities northeast of Fort Rock a few miles. Inflated lava, kīpuka, and lava ponds are found here, plus both ʻaʻā and pāhoehoe flows. Fort Rock Valley Historical Homestead Museum and the small community of Fort Rock are one mile south of Fort Rock State Natural