Contraction during cooling of thick, extrusive basaltic lava flows causes cracks perpendicular to the cooling surface, which form the polygonal columns of the Devil's Postpile in California. The columns average about 46 cm across. In thick slowly cooled flows the joints normal to the flow surface tend to develop in three directions at roughly 60 to each other and form multi-sided blocks that are cut off by the joints parallel to the flow surface. Jointing of this sort may attain a high degree of regularity. The horizontal joints divide the columns into a series of segments. Such columnar jointing is found in all types of lava flows, including ash flows. It is also conspicuous in dikes and other intrusions.