Sand of Mannin Bay

Mannin Bay is located in Connemara, in the west of Ireland. This place is noteworthy because the coast there is covered with light-colored and coarse grains that very much resemble calcareous pieces of corals. Is it really coral sand? In Ireland? That, I am afraid, is not possible. Corals prefer much warmer water.

This sand is locally known as maerl. Maerl is a type of coral sand (in the loose sense of the term) that is composed of calcareous pieces of coralline algae. Mannin Bay is only few kilometers away from the Dog’s Bay which is perhaps better-known because there is a nice white-colored beach and a parking lot for cars. Mannin Bay is somewhat harder to access. The road runs close to the coast but there is really no room for proper parking and one needs to climb over the fence to access the place. I, of course, did it because being a geologist who wants to see stuff means that you often need to park your car where you should not and go where you are really not supposed to go.

Mannin Bay is close to Dog’s Bay but it is even somewhat more special place. Sand of Dog’s Bay is finer and it is composed of many different biogenic grains (sponge spicules, clam shells, foraminifers, sea urchins, and gastropods). Sand of Mannin Bay is coarse and it is almost exclusively composed of pieces of coralline algae (Rhodophyta). Two of the most common species of coralline algae on the Ireland’s west coast are Phymatolithon calcareum and Lithothamnion corallioides. The algae lives less than 20 meters below the sea level. Living algae is reddish in color. Pieces of it get washed ashore during storms.

Pieces of branched coralline algae of Mannin Bay. I picked grains with a more characteristic shape for this picture. Width of view 6 cm.

Here is a close-up picture of Mannin Bay sand taken in situ.

Mannin Bay is covered with coarse biogenic sand composed of coralline algae.

A view towards the sea.

These black rocks are metamorphic, probably metamorphosed mafic igneous rocks.

Maerl forming terraced banks of a small stream.

Metamorphosed and folded metabasite of Mannin Bay.