Vertical columnar jointing forms as a result of cooling, particularly of lava. The joint systems associated with cooling are typically polygonal in nature due to the cooling introducing stresses that are isotropic in the plane of the layer. A good example of columnar jointing can be see at Giant's Causeway in Ireland.
The final period of the Mesozoic. This period spans 145.5-65.5Ma. The end of the Cretaceous was marked by the KT (now properly called the KP) extinction event, which saw the demise of the dinosaurs as well as several other groups such as ammonites.
GeologyRocks