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Rocks on the shore at Cushendall
The following information has been supplied by
Dr Michael J. Simms Curator of Palaeontology Department of Natural Sciences National Museums Northern Ireland Cultra, Holywood, Co. Down, BT18 0EU
We are very grateful for his assistance and for giving his permission to use his work.
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Cushendall Lifeboat Station
Two very different rock types can be seen on the shore on either side of the bay around the lifeboat station.
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View of Red Arch from Lifeboat Station This shows two different types of conglomerate separated by an erosion surface (i.e. flowing water scoured the top of the lower pebble beds before depositing the higher ones)
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View of Garron Point from Lifeboat Station |
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A slightly younger series of sandstones in which there are more angular pieces of rock, mostly schist, that had not travelled very far from their source and so haven’t had their corners worn off.
This is further south, towards the Red Arch itself. |
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Rhyodacite |
To the north of the slipway
On the shore below the caravan park is an exposure of a hard, blocky, yellowish-brown to pinkish-purple rock, often spotty in places.
This is a type of volcanic lava called Rhyodacite (pronounced Ryodaysite) and was erupted during the Devonian period around 400 million years ago. This is a very viscous type of lava that is produced today by volcanoes which erupt explosively, such as Mount St. Helens.
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South of the lifeboat station
The rocks are very different, though of similar age to the rhyodacite to the north. These are pebble beds, or conglomerates, which were deposited in an ancient river. The pebbles include several rock types, the commonest being rounded cobbles of quartzite, a very hard rock type, and pieces of Dalradian schist which is rather softer.
The largest of the cobbles are more than a metre across, showing that at times this river must have been very fast and turbulent. In places we can see some of the flatter cobbles overlap, like shingles on a roof. This can tell us the direction the water was flowing, with each pebble overlapping the one below in a downstream direction. The very rounded shapes of many of the quartzite cobbles shows that they have experienced a great deal of wear. |
Conglomerate Rock at Red Bay
This shows the conglomerates with large rounded cobbles near to the lifeboat station. |
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One possibility is that they were carried for hundreds of km along this ancient river from some distant quartzite mountains, but the large size of the pebbles makes this unlikely as the river would have been unable to transport large pebbles once it had leveled out on the flood plain. A more likely explanation is that the pebbles are actually ‘second hand’ and have been recycled from an older pebble bed in which the pebbles perhaps were formed on a beach.
This same process of ‘recycling’ of pebble is happening today at Cushendall, since many of the loose pebbles on the beach actually come from this ancient pebble bed. There are two lines of evidence that show that the cobbles in these ancient pebble beds are recycled. Firstly, some of the cobbles have been broken but the broken ends are still quite angular rather than rounded off. Secondly, and even more convincingly, a few of the embedded cobbles are themselves made of rounded lumps of an even older pebble bed.
All of the beds on the shore here have since been tilted by earth movements, caused by the collision of continental plates during the process of continental drift, and now dip steeply to the north-east.
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There are extensive Devonian rock outcrops further along the coast to the north-west of the town, with car parking adjacent to this beach. Most of these rocks are conglomerates, dipping steeply to the north-east, containing a range of different pebble rock types.
On Salmon Rock, immediately opposite the start of the coast path, the conglomerate contains boulders up to a metre or more across though most are much smaller. The boulders and pebbles, set in a yellow or purple matrix, range in colour from brick-red, through greenish-yellow, to blue-grey or purple; many have a spotted appearance caused by small crystals or gas bubbles in the lava.
One especially large boulder, more than 2 metres across, seems to show all of these colours. These multi-coloured pebbles and boulders are of a type of volcanic rock called andesite and this type of volcanic-dominated pebble bed is often called an agglomerate. There are also a few well-rounded quartz pebbles.
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Salmon Rock, Cushendall |
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Cushendall Rock Strata
Thin beds of sandstone and mudstone deposited on an ancient flood plain, with conglomerates containing volcanic blocks forming Salmon Rock in the background
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At low tide it is possible to walk further north along the shore, to where a set of steps descends to the beach. In this area the rocks are coarse red sandstones which are about the same age of have the same north-easterly dip as the conglomerates further south.
They show cross-bedding, formed on underwater dunes, and occasional bands of pebbles, and were deposited in an ancient river system rather less fast flowing and turbulent than that which formed the conglomerates further south.
Occasional isolated pieces of vesicular (full of gas bubbles) rhyolite, a type of lava erupted from explosive volcanoes, occur scattered through the sandstone; these represent volcanic ‘bombs’ thrown out of a nearby volcano. Within the sandstone there is also a thin continuous band of very fine-grained red and yellow tuff (volcanic ash) this passes upward into coarser tuffs and then into agglomerate/conglomerate.
Clearly there was an explosively active volcano somewhere nearby when these river sandstones were being deposited. |
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Cushendall shore to the South |
Cushendall shore to the North |
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Arial view of Red Bay Lifeboat Station |
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