Cay Sandstone or “Beach Rock”

We are used to thinking of rocks as ancient structures that have been in place for millions of years but, of course, rocks are in the continual process of being formed. An example might be the way rivers carry erosion sediments downstream to form layers on the beds of seas, lakes, and lagoons. Or erupting volcanic lava solidifying on contact with air or water. On the coastline of Queensland in Australia the most easily visible type of present-day rock formation is that of cay sandstone, commonly called “beach rock”.

Beach rock forms very rapidly. It happens in warm shallow water close to coral reefs, where the combination of heat and evaporation, an abundance of dissolved calcium from pieces of coral and seashells, and the addition of phosphates from bird guano, lead to a cementing of all the loose fragments together to form hard concretions of rock. This is such a rapid way of rock building that it is sometimes possible to see man-made objects included in the concretion – apparently soft drinks cans have been recorded. More commonly seen are pieces of coral (sometimes still coloured), sea shells, and the impressions of plant remains such as Pandanus fruits.

The photographs in this post were taken mostly on the sheltered shore of Normanby Island where the beach rock layers form an almost continuous link and low-tide walkway to the neighbouring island. Large slabs of beach rock are prone to break off from the layers and rest on the shore. In other places deep deposits of algal-covered rock have started to wear into depressions and hollows that form new habitats for marine gastropods and crustaceans.

I travelled to Normanby Island on a tour with Cruise and Dive Frankland Islands.