Hálsanefshellir sea cave, Reynisfjara Beach, and the Gardar Cliff in Iceland. The geology is made of basalt columns. From the beach the arched cliff of Dyrhólaey across the sea.

The Gardar Cliff has a pyramidal structure formed by the columnar basalt at this location (Photograph by Dianna Gielstra). As magma cools. Slower cooling of the magma over time causes the surfaces of the columnar joints to contract and crack along the surfaces. The contraction takes the form of one of the most efficient shapes in nature, polygons. This form is known as a colonnade.

Columnar Basalt

Repetitive wave action that eroded the rock and formed a cave, known as Hálsanefshellir 

Hálsanefshellir Cave

These tidal environments are high energy, and the waves create a large array of features. The consistent impact of waves against the joints and cracks in the rock loosens the material and forms sea caves. Continued erosion forms arches, and when these collapse only the** pillars** remain (National Park Service, n.d.). Reynisdrangar stacks have similar geology as Reynisfjara Beach. The features are approximately 66 meters in height (Katla Geopark,n.d.).

 Reynisdrangar Stacks

These tidal environments are high energy, and the waves create a large array of features. The consistent impact of waves against the joints and cracks in the rock loosens the material and forms sea caves. Continued erosion forms arches, and when these collapse only the** pillars** remain (National Park Service, n.d.).

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