Geology for National Schools in Ireland

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Rocks, like cakes, often have layers, as here at Mellon Point on the Shannon. Like cakes, rocks start out soft and, like cakes, lots of the stuff in their 'cake mix' can still be seen (but instead of fruit, it's usually small shellfish and snails, and beetles). We call these creatures turned to stone, fossils.


The bright blue area shown here is what is known in the geology of Ireland as Ballysteen limestone, and it stretches from the Maigue to the Deel. The darker blue area is a different kind of limestone (and is much better for finding fossils). Fossils tell us a lot about the kinds of creatures that inhabited the Earth before any humans came along. The reddish stuff is sandstone, which doesn't have fossils, and the pink patch there to the right is Carrigogunnell, which once was actually a volcano, and much too hot for fossils.


Our first fossils were found here, at Bushyisland, where the bedrock (grey) that is under the river silt (yellow) and under the ice-age gravel (blue), shows up on the shore. The fossils are surprising.


Don't these look like the honeycombs you see in a jar of real honey? They are the fossils of 'honeycomb corals', called that because of their appearance. Corals, in Ireland? Yes, because Ireland was once a long way south, in warm coral seas.


This is a loose piece of coral found at Bushyisland, just like this. You should not break off or remove a coral unless you are an expert collector, and if removed it should be kept in a known collection (this one was photographed where it lay on the shore, on the back of the collector's jacket).


Honeycomb corals liked to move in on one another, forming floppy colonies like this one near Beagh Castle.


These marks in the rock at Shannonview, across from the Deel Boat Club, are the tracks of mud worms. But worms don't do rock climbing, you say? Of course not, but the rock has actually been turned that way up by some prehistoric upheaval, perhaps an earthquake, and the rock was once mud, at the bottom of a shallow sea.


Ballynash millipede fossil

Ballynash millipede fossil

The fellows above, when they were alive, looked like the chap below, who is a millipede ('thousand legs'). They are known as detritivores, and they eat up dead leaves and other decaying vegetation. They are, in fact, an important part of nature's own recycling team.


Here is a whole crew of them, at Ballynash, on their way to work (or lunch – same thing). Whatever stopped them in their tracks?


Photo: Hook Area Tourism Ltd.

On this rock, from the Hook Head area of Co. Wexford, you can see a little piece of a millipede fossil, like our friends at Ballynash, over there on the left. You'll notice that the big, round holes seem to spiral round and around, right to the bottom. That's a sure sign that they were snails. The snails are gone, only their shape remains.


At Ballynash too, there are odd looking round holes and also raised round shapes in the rock. Probably not snails: the Ballynash rock is Waulsortian limestone, made from the mud banks of shallow, warm seas, and these are the marks, most likely, of sea lily roots. Sea lilies were (and are) also detritivores – they could and can eat!


A sea-lily fossilised root. It was more than a plant and had arteries and nerves, like people (but it's 'blood' was sea water). There's a little round ring in the B&W photos too. Spot it?


A sea lily fossil: do you notice the 'rings' on its stem?

photo: Kaj R. Svensson

This is a modern cousin


These are actually parts of fossilised sea lily stems (some of them grew very big).


Photo: UKRI British Geological Survey

How sea lily fossils might look when first found. The big tube in the middle was a sea lily stem. We said they were detritivores. What they ate (and still eat) was plankton, tiny organisms, and decaying materials that fall to the sea floor. How? They use their arms (that look like palm branches) to sieve and catch microscopic material and pass it down into the mouth of the sea lily. Sea lilies are related to starfish.


That creature there under the chop is a sea cucumber ̵ you can see how it gets its name; it's a delicacy of Korean cuisine


Photo: Humboldt State University CA

A fossilized sea cucumber: it was just a big, fat worm (they are still around) and this one was a detritivore, like the millipedes, eating up dead plants in the mud banks that later turned into rock.


Courtbrown Point, where there is a lot of Waulsortian limestone, and where the next fossil was found.


That's the fossil. Something that may have been a shell was curled around it, and the creature itself is curled all the way right around (look closely at the middle). Including outer ring, it's about 6-7 cm across. Could it have been a sea cucumber worm? Imagine!


These are what are known as rugose or wrinkled corals and, as you can see, they have left their shapes in the rock (the rest of the coral is buried in the stone). Now, as you know, we have corals here on the Shannon and the Deel too, and some marks found on our rocks look a bit like these. So, next time you are there (or down at Fenit – great for fossils) keep a sharp eye out!


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