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The Fault Lies Here: Ballintoy Harbour

A cache by @rcane Send Message to Owner Message this owner
Hidden : 05/15/2016
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3 out of 5

Size: Size:   other (other)

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Geocache Description:

Ballintoy Harbour has come to worldwide attention as the location for the Iron Islands in the HBO series Game Of Thrones. The area is picturesque and photogenic but also has a rich geology and geomorphology. Near the harbour are several interesting geological features; a volcanic plug, indications of raised sea levels and well-defined local faulting, while a short walk to the west lies a geologically rich landscape of basalt sea stacks and rock arches. A lot of geology in a small area.

The Geology Of Ballintoy

Walking east from the car park down a rough track and across a narrow beach you can stroll alongside the smooth edges of a huge, dark, bare basalt plug marking the position of a local volcanic vent called the Bendoo plug.

Volcanic plugs

A volcanic plug is a landform created when volcanic magma hardens inside a vent on an active volcano. They are sometimes called volcanic necks or puys. The plug is revealed when surrounding land is eroded. As it forms a plug may cause an extreme build-up of pressure if gas-charged magma is trapped beneath it. This sometimes leads to an explosive eruption, but if the explosion does not occur then the volcano becomes a solid block made of hard material. The plug is usually harder than the surrounding rock so that erosion may remove the surrounding rock while the erosion-resistant plug remains. This is what produces the distinctive upstanding landform.

Walking back towards the car park and along to the end of the harbour you can look across to the chalk/limestone platform and observe a small but clearly visible example of local faulting. Here the near horizontal bedding in the chalk begins to dip northwards. This is the result of the friction generated by the downthrow to the north and is known as “fault drag”. A more dramatic example of faulting can be observed from the top of the lime kilns backing the car park from where the two main formations, the chalk and the basalt, can be easily identified. It can be seen that these rocks occur at different levels, a sure indication of faulting. The Ballintoy fault runs westwards through the harbour and across Whitepark Bay to Portbraddan. To the north of this line the basalts are at sea level while to the south the chalk is on top, the structures to the north are downthrown in relation to those to the south, a vertical movement of some 100 metres.


A fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock mass movement. Faults form when stress from underlying rock movements causes the more brittle surface rock to crack or slip. The result is a discontinuity in the landscape that can be either laterally displaced, sunken or lifted up relative to the surrounding terrain. Fault lines can be very short and affect only a small area, or they can run for hundreds of miles. The two sides of a non-vertical fault are known as the hanging wall and footwall.

Heading westwards across the car park you will find an excellent example of a raised beach with a couple of sea caves now clearly well above sea level. This results from the rise in sea level when the ice melted and the recovery of the land which followed later.

Raised beaches

A raised beach is an elevated area of sloping ground sitting above the present tide line. In the past this area was at sea level. During an Ice Age the massive weight of ice bearing down on a landmass caused it to sink. Over time, as the earth's temperature rose and the weight of ice decreased, areas of land began to slowly rise back out of the sea. This ‘bounce back' motion - the localised change in sea level relative to the land - is known as isostasy, or isostatic uplift.

Walking on past the two cottages the geological trail continues as part of the Causeway Coast Way for the next ¾ km through a series of raised beaches and stranded sea stacks, a walk well worth taking, however there is an area of bog, a small stream and two stiles to be negotiated so it is not suitable for wheelchair users or childrens' buggies. Be careful with children and dogs if walking out onto the limestone plateau at the eastern end of the harbour as there are sheer drops into the sea, an especially unannounced one being the 'blow hole' cut into the rock by wave erosion from which a particularly large incoming swell has been known to drench the unwary.

Please email or message me the answers to the following questions. There is no need to wait to log your find, I will get back to you if there are any problems. Any logged finds for which I have not received a message/email containing the correct answers will be deleted, as will any logs containing spoiler photographs.

1. From the sign beside the stone cottage at the harbour itself, how long ago did faulting and the outpouring of volcanic lavas help create the distinctive landscape at Ballintoy?

2. Describe the particular appearance and surface texture of the rocks to be found at N 55° 14.643 W 6° 21.966.

3. What feature can be found at N 55 14.642 W 6° 22.969?

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