Cork harbour is large enough to have a significant number of islands, many of which are populated including Haulbowline Island, which accommodates the headquarters of the Irish Naval Service, Spike Island which was formally a prison island and Great Island, the largest island in the Harbour with a population of over 10,000 and includes the town of Cobh, clearly visible across the bay from the Earthcache location.
Geology
The overall topography of the harbour is strongly defined by the underlying geology.
The rocks at the surface around the bay are typical of most of County Cork ranging from Devonian Old Red Sandstone (formed about 419 to 358 million years ago) which were originally laid down in semi arid desert conditions when Ireland's land mass was near to the equator and Carboniferous Limestone (358 to 219 million years ago). The name Carboniferous means "coal-bearing" and derives from the Latin words carbo (coal) and fero, fers (to carry).
According to Dr. Ed Jarvis, (Lecturer in Geology, University of Cork), Old Red Sandstone gets its name because it's old, it's red and yes, it's a sandstone! The familiar red colour of these rocks arises from the presence of iron oxide, deposits of which are closely associated with the erosion of the Caledonian Mountain chain which ran across Ireland.
The Devonian and Carboniferous rocks were subjected to intense tilting, folding and faulting which began at the end of the Carboniferous period which is known as the Variscan Orogeny. These folds in the rocks run in an east-west direction and are the main reason for the harbour's unusual shape and size.
The entrance of the harbour is relatively narrow, being about 1.5km wide and was originally formed by an anticline, (a tight fold) of rock where some old red sandstone rock can be seen at the surface. A fault formed in the middle of this fold, allowing the river to carve through a channel and eventually form the harbour we see today. Another fold to the north forms the island of Long Island on which the town of Cobh can be clearly observed from the Earthcache location.
Here at Ringaskiddy, we see the carboniferous Limestone coming to the surface and can clearly be seen when the tide is out forming ridges of rock on the beach. While there isn't much coal in the rock strata beneath the bay, not far out from the bay's mouth, Ireland's only natural gas producing rigs are found.
Immediate geology at the location
The beach here at Ringaskiddy is also interesting from a far more recent geological point of view. A mere 20,000 years ago (a tiny period of time geologically speaking) this entire area - and indeed most of Ireland was covered by a thick layer of ice. In fact, this area was very close to the southern point of the ice cap that formed during the last Ice Age. As the ice retreated, it left a huge amount of evidence on its departure – much of this can be seen here.
Take a look at the cliffs to the south of the beach. Geologically, these are mere youngsters and were formed roughly 50,000 years ago from what is known as till. Till is formed from unsorted glacial sediment – materials that were eroded by the glacier as it moved over the land beneath it, carried along as the glacier flowed and then deposited when the glacier retreated. As you examine the cliff face you'll see a series of layers of soft clay interspersed with boulders and pebbles. Many of these boulders are rounded suggesting they were formed by the flow of a river or sea but if you look carefully, some also have striation marks which is evidence that they were dragged by the glacier and eroded against other rocks. The boulders and pebbles are also of diffent rock types clearly indicating that they are originally from other locations.
There is another clear piece of evidence from the last glacial period that can't have escaped your attention when you arrived here. There is a curious large boluder in the sea, with it's bottom now eroded, to the south of the beach, which can be reached by foot when the tide is completely out. This boulder looks out of place, which it is! This boulder is what is known as an erratic – a rock that was transported by a glacier and deposited when the glacier retreated. Erratics can range from massive rocks the size of houses to small boulders – but the clear evidence that they were transported there is the fact that they don't match the local rock type. Indeed, if you look, there are a number of smaller erratic stones in this area too.
Logging your find
Once you have visited the location and have the answers to the questions below, you can log your find. You will however need to send me seaparately the answers to the following questions reasonably shortly after logging your find otherwise the find log may be deleted. If answers are included in the log entry, the log will be deleted.
Please click on the "I want to send my email address along with this message." tag - it makes it so much easier to respond.
1. As you enter the beach, turn to your right and take a short walk southwards (about 100 paces at most). This should be possible in most tidal conditions apart from extreme spring tides. Where you first see a small patch of exposed till cliff face, estimage the height of the cliff from beach floor to top at this location.
2. At this location, what is the predominant colour of most of the pebbles and boulders found in the cliff face? Can you think of an explanation as to why the rocks are this colour and why they are found here - where do you think they came from?
3. [Optional] Take a photograph of something geologically significant at the location and include it with your log entry. It can be big or it can be detailed!
Here is a video of Dr. Ed Jarvis talking about the harbour's geology while on a boat trip out in the harbour.
