Our meeting point was Garryvoe on Saturday October 6th, 2007
at 11.00am and our leaders were Prof. Ken and Dr. Bettie Higgs,UCC. We drove approx. two miles along the coast in an easterly direction to Ballywilliam Beach.
Stop 1:
At the back of the south east facing sandy beach is a boulder clay cliff. It is approx 4m high and contains unconsolidated material. This material is rounded and includes sandstones, some Cork marble, chert and Ailsa Craig granite clasts. This is part of the Quaternary deposits of the last ice age.
The clasts form a coarser layer which has a wavy pattern approx. 2 metres up from the base within the boulder clay cliff. The material has been sorted and is rounded. At between -22 degrees and 0 degrees the ground freezes and contracts. Frost heaving occurs. Part of this process involves stones moving upwards onto the surface. This area was not covered by ice during the final Quaternary period but the mean annual temperature was as low as -5 degrees and soils were frozen. Frozen subsoils are known as permafrost.
The upper few metres of permafrost may thaw during the warm periods and the ground expands and freezes again in the cold periods and the ground contracts. Frost heave is a process whereby silts and clays expand and cause stones to move upwards towards the surface.
On undulating ground this could result in the material forming this wavy pattern as at Ballywilliam. This is called involution and appears as ‘patterned ground’.
Stop 2:
There is a pattern of mounds or collections of rounded stones in the cliff face occurring about half a metre from the cliff top. These mounds occur along the cliff at intervals of approx. three metres or more.
Question : Are these mounds man-made or a natural deposit?
Theory 1
A possible further development of this patterned ground is the accumulation of stones in mounds as the uplifted stones roll down a sloping surface and collect in hollows.
Theory 2
Man made: drainage gully.
Farmers wishing to drain the land may have dug ditches along the surface and filled them with stones from the beach. Evidence of this was the presence of a plastic bright yellow wavin pipe protruding from one particular mound and draining out onto the beach.
Stop 3:
Continuing eastwards for 400 metres approx. we came to an outcrop of bedrock. This is the Garryvoe conglomerate facies of sandstone. The dip is almost vertical at 80 degrees and the strike is East-West. This is part of the syncline that is completed across the bay at Ballycotton. It includes a layer of marine gravels sourced from off shore bars and these gravel layers increase in thickness to 30cm to the east. Some layers contain evidence of wave rippling.
Stop 4:
Approx 50 metres further east is a deposit of black mudstone. These are very dark in colour and are fine grained. These were formed in deep marine environments of the Upper Devonian. We gradually came to fine grained sandstone deposited when this sea became more shallow during the Lower Carboniferous period.
We finished our fieldwork at this site and then made our way back towards the carpark in glorious sunshine passing again the puzzle of the stony deposits. A very stimulating trip. Thank you Bettie and Ken.
Rosemary Murphy, B.Sc., CGA member.
(Photos by Philip Meaney, CGA member.)
A general view of Ballywilliam beach looking west with the boulder clay cliff in the background. The unprotected cliff is being constantly eroded.
The up and down/wavy layer of clasts in the boulder clay. This was caused by frost heave in the permafrost during the final part of the Quaternary period.
Another view of the coarse layer.
A CGA member examines the mound of beach stones in the cliff face
Erosion of the boulder clay in the cliff face
Garryvoe conglomerate sandstone. The syncline is completed across the bay at Ballycotton
Other views of the rocks at the eastern end of Ballywilliam beach
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