The Causeway you are referring to is likely the Giant's Causeway, a famous rock formation in Northern Ireland. It is made up of basalt columns, which are formed by the rapid cooling and contraction of lava flows.

Here's a more detailed explanation:

* Volcanic origin: The Giant's Causeway was created millions of years ago when a volcanic eruption spewed molten lava onto the Earth's surface.

* Cooling and contraction: As the lava cooled and solidified, it contracted, causing it to crack and break into distinct columns.

* Hexagonal shape: The majority of the columns at the Giant's Causeway are hexagonal in shape, although other shapes like pentagons and heptagons can be found. This is due to the efficient way in which a hexagonal pattern distributes stress within the cooling rock.

Therefore, the Giant's Causeway is primarily a volcanic rock formation, specifically formed by basalt columns.