Columnar Jointing (above)
Carlo Forte (April)
Here is a good example of columnar jointing on the ridge of Crib y Ddysgl.
The rock on the top of the ridge is an upper rhyolitic tuff formation.
The regular hexagonal patterns to the columns and the formations occur due to stress lava cools. 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.
The division of the colonnades is the result of slow cooling from the base upward and rapid cooling from the top downward.