Delta Journal

by Bob Thomas

If you spend much time on the beaches of Louisiana, you are bound to find odd looking hardened objects that appear to be stone. But, rocks on a beach? That doesn’t make sense on the Gulf coast.

These normally flat stones, called lithophide stones or beach rocks, are usually found in coastal deposits, usually washed out on the beach front. They are an indication that the beach is eroding and that the spot where they are found used to be the rear of the barrier island.

Lithophide stones are formed in wash-over zones at the rear of barrier islands, at the interfaces of beach/marsh and water/sand. Methane is produced in marsh areas. When the methane reacts with shells in the presence of fine sands, a carbonate cement is formed. This becomes the lithophide stone.

They are usually irregular in shape, though almost always flat. Holes are usually caused by the stone forming around something that has either decayed, fallen, or eroded away. Shea Penland has seen them growing down Ghost Crab burrows and around coke bottles.

These stones may also form as a result of iron interacting with its surrounding environment.