About 60 million years ago, late in the splitting up of Pangaea, the North Atlantic Ocean began to open. The rifting was accompanied by extensive vulcanism -- the evidence of which can be found in basaltic rocks from Greenland to Northern Ireland. These basaltic flows are exposed in multiple layers along the eroding edge of the Antrim Coast, nowhere as well as at the Giant's Causeway.
The slowly cooling basalts form the familiar polygonal columns (rarely regular polygons and not always hexagonal). Their broken tops form the shore platform and the causeway itself. The nature of these broken surfaces is to bow either either up or down - the ones that curve down form puddles.
The Causeway is mainly a story about platforms and sea cliffs and igneous geology. Not about beaches. But, in the midst of all the columns and the tourists, I think there really is a beach. This big pile of broken and rounded columns forms a distinct berm at the head of a trough in the platform. I think this is a beautiful example of boulder berm (try to ignore those huge dinosaur-size boulders - not sure what erosional process forms those). We'll see other examples later on this trip, but for another example (not basalt) from the other side of the Atlantic, see August 2010: Schoodic Peninsula.