Straight, regular columns are called colonnade. Irregular, fractures columns are called entablature. The lava contracts as it cools, forming cracks. Once the crack develops it continues to grow. The growth is perpendicular to the surface of the flow.
Entablature is probably the result of cooling caused by fresh lava being covered by water.
The flood basalts probably damned rivers. The division of colonnade and entablature is the result of slow cooling from the base upward and rapid cooling from the top downward. 1931 postcard of Devils Tower, Wyoming, a shallow intrusion that formed columnar jointing as it cooled. Possible mechanism for the formation of columnar jointing at Devils Tower. Isotherms are layers with the same temperature.