September 3rd  - Ballintoy Harbour, Geology

Leader: James Rutherford. 


The site with so much basic geology to offer in easy steps: 

Ballintoy Harbour is not just an impressive North Coast beauty spot, it is also a Mecca for anyone who is tuned in to ROCKS; in fact these two aspects enhance each other. Here the forces of erosion expose the spectacular geology clearly and in an accessible setting, encouraging the student to investigate.         

We began with the landscape as viewed from the top of the lime- kilns. The two main formations, the chalk and the basalt, were easily identified. We noted that these rocks occurred at different levels, a sure indication of faulting. We could define the major fault, the Ballintoy fault (Fig 1), running westwards through the harbour, 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; in other words the structures to the north are downthrown in relation to those to the south, a vertical movement of some 100 metres. 

Just to the east, above the harbour it was observed that 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”. 

Behind the kiln we examined a minor dolerite dyke (averaging 1.5 metres wide). Being a small dyke the heating of the chalk was limited to about 15 cm. but the marbling effect was clear enough. In the car park we examined a sea cave, 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. The cave was eroded out along the line of weakness on the fault line. The movement along this line resulted in shattering the chalk on both sides of the fault. This is known as “fault breccia”. 

A short way west of the cave the raised beach displays several sea stacks composed of basalt and now stranded above sea level. Farther west there is a good example of rotational slip (a common feature along the Antrim coast road). Part of the chalk cliff has broken away and slipped down on to the raised beach. Rain percolates down through the joints in the chalk, lubricates the impermeable Lias clay below and part of the cliff slides away under its own weight. The bedding planes in the displaced member are now steeply tilted.

Opposite the two cottages a layer of reddish material is visible between two lava flows. This results from oxidising of the iron content, a common element in basalt. Adjacent is a small volcanic vent. The erupted material is shattered and contains fragments of basalt, chalk, lias and greensand. Unfortunately fresh sand has been added in a vain attempt to replenish the beach but the vent is now partly obscured. 

Finally we turned our attention to the part of the beach east of the harbour. Here the two features of interest are the Bendoo plug and the effect of sea erosion on a low chalk cliff. The plug is dolerite is roughly cylindrical, is some 350mm across and shows examples of rough hexagonal columns. If a contact metamorphic zone is present, on this occasion it was covered by beach sand. On the seaward side of the plug is the chalk cliff. On its surface is a dwindling number of small belemnite fossils (vandalism) and one large specimen of an ammonite 50 cm across. On the seaward side are some good examples of sea erosion features. The sea has concentrated on joints to open up narrow trenches, in some instances ending in blow-holes. A high tide and a rough sea can generate impressive explosions as the sea is driven into the joints and the water spurts upwards, drenching the unwary.

Quite a lot of geology in a small area.

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Fig 1

Belemnite Fossil

Ballintoy Blowhole

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