Sediment Clasts and Ventifacts from the North Coast of Northern Ireland Author(s): Peter Wilson Source: The Irish Naturalists' Journal, Vol. 23, No. 11 (Jul., 1991), pp. 446-450 Published by: Irish Naturalists' Journal Ltd. Stable URL: http://www.jstor.org/stable/25539614 . Accessed: 15/06/2014 00:47 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. . Irish Naturalists' Journal Ltd. is collaborating with JSTOR to digitize, preserve and extend access to The Irish Naturalists' Journal. http://www.jstor.org This content downloaded from 91.229.229.101 on Sun, 15 Jun 2014 00:47:53 AM All use subject to JSTOR Terms and Conditions
Transcript
Sediment Clasts and Ventifacts from the North Coast of Northern IrelandAuthor(s): Peter WilsonSource: The Irish Naturalists' Journal, Vol. 23, No. 11 (Jul., 1991), pp. 446-450Published by: Irish Naturalists' Journal Ltd.Stable URL: http://www.jstor.org/stable/25539614 .
Accessed: 15/06/2014 00:47
Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp
.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].
.
Irish Naturalists' Journal Ltd. is collaborating with JSTOR to digitize, preserve and extend access to The IrishNaturalists' Journal.
http://www.jstor.org
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I am grateful to tne staff of the Wildlife Service for assistance with field work and
provision of additional records, particularly, C. Saich, P. Behan, P. Smiddy, T. Ryan, A.
Hogan and D. Scannell. I am also grateful to colleagues at University College Cork,
especially M. O'Sullivan and A. O'Mahoney who helped draft the main map. Dr P. S.
Giller and Dr T. C. Kelly kindly read drafts of this paper and provided critical advice. Dr C.
F. Mason and Dr S. M. Macdonald (University of Essex.) took time to discuss the standard
survey technique efficiency. P. Chapman (Vincent Wildlife Trust, London) provided 1980-81 survey records for the Blackwater Catchment and R. Lunnon (TCD) further
clarified V.W.T, records for the River Bride. J. Green and R. Green (Vincent Wildlife
Trust, Scotland) provided additional information on otter surveys in Scotland.
References
Andrews, E & Crawford, A. K. (1986) Otter survey of Wales 1984-85. Vincent Wildlife Trust, London.
Chapman, P. J. & Chapman, L.'L. (1982) Otter survey of Ireland, 1980-81. Vincent Wildlife Trust, London.
Conroy, J. W. H. & French, D. D. (1987) The use of spraints to monitor populations of Otters. Symp. zool. Soc. *
Lond. 58: 247 - 262.
Crawford, A., Evans, D., Jones, A. & McNulty, J. (1979) Otter survey of Wales 1977-78 Society for the
Promotion of Nature Conservation, Nettleham, Lincoln.
Gormally, M, J. & Fairley, J. S. (1982) Food of otters Lutra lutra in a freshwater lough and an adjacent brackish
tough in the West of Ireland. J. Zool., Lond. 197: 313-321.
Green, J. & Green R. (1980) Otter survey of Scotland 1977-79. Vincent Wildlife Trust, London. ___ &-(1987) Otter survey of Scotland 1984-85. Vincent Wildlife Trust, London.
Jefferies, D. J, (1986) The value of otter (Lutra lutra) surveying using spraints: an analysis of its successes and
problems in Britain. J. Otter Trust I (9): 25-32.
Kruuk, H,, Conroy, J. W. H., Glimmerveen, U. & Ouwerkerk, E. J. (1986) The use of spraints to survey
populations of otters Lutra lutra. Biol. Coriserv. 35: 187-194. ?? & Conroy (1987) Surveying other Lutra lutra populations: a discussion of problems with spraints. Biol.
Conserv. 41: 179-183.
Kyne, M.J., Smal, CM. & Fairley, J.S. (1989). The food of otters Lutra lutra in the Irish midlands and a
comparison with that of mink Mustela vison in the same region. Proc. R. Ir. Acad. 89B: 33-46. ,?-? Kyne, M, J. & Fairely, J. S, (1990) A summer survey of otter sign on Roundstone Bog, South
Connemara. Jr. Nat. J. 23: 273-276.
Lemon, E.J., Chanm, P.R.R& Jefferies, D. J, (1980) Otter survey of England 1977-79. Nature Conservancy Council, London,
McFadden, Y. M. T, & Fairley, J. S. (1984) Food of otters Lutra lutra (L.) in an Irish limestone river system with
special reference to the crayfish Austropotamobius pallipes (Lereboullet). J. Life Sc. R. Dubl. Soc. 5: 65-76. .
Maedonald, S, M. & Mason, C. F, (1982) Otters in Greece, Oryx 16: 240-244.
Mason, C. F. & Maedonald, S. M, (1986) Otters: ecology and conservation. Cambridge University Press.
Cambridge. ___. & _^. (1987) The use of spraints for surveying otter Lutra lutra populations: an evaluation. Biol.
Conserv. 41: 167-177.
Murphy, K. P. & Fairley, J, S, (1985) Food and sprainting sites of otterson the west coast of Ireland. Ir. Nat. J. 21: . '.' 477-479. .
SEDIMENT CLASTS AND YENTIFACTS FROM THE NORTH COAST OF NORTHERN IRELAND
Peter Wilson
Department of Environmental Studies, University of Ulster at Coleraine, Cromore Road, Coleraine, Co Londonderry BT52 ISA
Sediment clasts and ventifacts have been found at separate locations along the north coast of Northern Ireland. Although similar features have been recorded from various environments in many areas of the world, this is believed to be the first report of their occurrence from the Irish coast.
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Sediment clasts are fragments of cohesive sediment that have broken away from their parent sedimentary body. They are sometimes reworked by water (waves or streams) and as a consequence usually undergo size reduction and rounding. They may eventually disintegrate or be buried and thus preserved in the sedimentary record. The most commonly reported types of sediment clast are armoured and unarmoured mud (or clay) balls (Haas 1927, Bell 1940, Kugler and Saunders 1959, Dickas and Lunking 1968, Karcz 1969,
Stanley 1969). The former consist of an inner core of mud surrounded by an 'armour* of coarse sand grains and fine gravel. Unarmoured mud balls lack a coating of sand and gravel. The sediment clasts reported here are analogous to unarmoured mud balls except that mud or clay is not their major component.
At Port Ballintrae, Co Antrim (C923421), sediment clasts have been found on the beach beneath cliffs composed of Late Pleistocene deposits. The clasts are fashioned by waves from larger masses of material that have collapsed onto the beach as a result of cliff undercutting. The morphometry of the clasts was determined by collecting a sample of 158 individuals, a selection of which is shown in Plate 10A, For each clast the long (a), intermediate (b) and short (c) axes were measured; the b axis was used as a representative indicator of clast size. The degree of clast flatness, which can be regarded as the inverse of
sphericity, was assessed using Wentworth's index ? a + bile (Wentworth 1922) and roundness was estimated using the formula recommended by Dobkins and Folk (1970) ?
Rs/Rr X 100, where Rs is the radius of the sharpest corner and R, is the radius of the largest inscribed circle measured in the maximum projection plane of the clast.
The results of these determinations are summarized, in Fig. 1 as a series of dispersion diagrams. Clast size ranges from 1.9 to 8.0cm, although 88% of clasts occur within the narrower range of 2 to 5cm, The size distribution is positively skewed and has a median value of 3.5cm and a mean value of 3.8cm. Flatness values range from 1.5 to4.4 (A value of 1.0 indicates a perfect sphere (a
- b - c) and departures therefrom indicate increasing flatness). The lowest flatness value of 1.5 derives from a clast with axial ratios of 1,0:0,75: 0.58; thus all clasts depart significantly from spherical. The high degree of clast flatness is
essentially inherited from the parent sedimentary body (see below) rather than related to
contemporary wave action. Roundness values have a wide range from 10 to 83, although only 20% exceed 50. (A value of 100 indicates perfect rounding in the maximum projection plane (Rs
= Ri) and departures therefrom indie ate increasing angularity). The variability in
clast roundness is illustrated by Plate 10A; most clasts are best considered as either sub-rounded or sub-angular.
Three clasts were disaggregated, thoroughly dispersed in dilute Calgon solution and
analyzed for their grain size characteristics by a combination of dry-sieving and
pipette-sampling techniques. The results reveal virtually identical size distributions; sand
(?10 to +40) is the dominant component (49-51.5%) with slightly less silt (+40 to
+90,40-43.9%) and relatively small amounts of clay (>+90,6.1-10%). Between 61,6% and 65.2% of the clasts are composed of material falling within a range of 30 units from
+20 to +50; therefore the clasts are dominated by fine and very fine sand and coarse silt.
Only one clast yielded particles <-10 but these were of negligible proportions (0.1%). In
Plate 10A coarse particles can be seen projecting from the surfaces of several clasts. These are not'armour' particles added by accretion, rather they represent original constituents of
the clasts. The CaC03 content of the three clasts, as determined by calcimeter, was
6.9-9.5%.
The grain size analyses indicate that the clasts differ from those described previously.
Although grain size characteristics have not been determined in other studies of sediment
clasts, it is apparent from descriptions that clay is the dominant size component. This fact,
coupled with the high degree of sphericity, has given rise to the term 'mud or clay balls1.
The Port Ballintrae sediment clasts may therefore represent a rather unusual example of the
phenomenon. The degree of cohesion within the clasts is rather weak; they are easily
disaggregated by gentle hand pressure and by water. Cohesion is probably provided by the
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imu''.i.ii.i|j|]!i'.ui|i i'-^j iiii^ij,,r"i.iTil^i...iiiivi, I . _.. ., *??n ,.?,.._._.^,*^^._.^?^?.v* *
>late 10. A: Sediment clasts from Port Ballintrae showing variability in roundness. Coarse >articles can be seen projecting from some surfaces. B: Ventifacts from Portstewart.
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Figure 1. Dispersion diagrams for sediment clasts size, flatness and roundness. Each dot represents one clast.
small clay and CaCO.i components and compaction by overlying sediment may have also
assisted this process. Interestingly, the basal sediments exposed in the cliffs at Port Ballintrae are glaciomarine muds (Eyles and McCabe 1989), but sediment clasts derived therefrom have not been found; it is from collapse of the overlying thinly-bedded marine sands that the clasts are being produced. The flatness of the clasts is a reflection of the thickness of individual sand beds which are up to 4cm thick.
The sediment clasts are ephemeral features of the beach environment. They are
produced rapidly during storm conditions or high spring tides when effective cliff
undercutting occurs. Rapid size reduction and eventual disintegration of the clasts is
promoted by an abundance of basalt boulders against which they are abraded and fractured and they are therefore unlikely to be preserved in the sedimentary record.
Ventifacts
Ventifact is a general term for any rock which has been shaped to some extent by the abrasive action of wind-blown sand. Typically they have flat or concave facets that meet
along a sharp ridge or keel. The number of facets is used to describe them as either *ein-\ 'zwei-' or kdreikanter' (or ridged ventifacts), or pyramidal and multi-facetted ventifacts.
They are most commonly reported from desert environments (e.g. Needham 1937, Sugden 1964, Lindsay 1973, Calkin and Rutford 1974) but can occur in other environments where a
supply of sand and strong winds prevail (e.g. King 1936). In northern England ventifacts have been described from Permo-Triassic and Quaternary sediments by Thompson and
Worsley (1967). At Portstewart, Co Londonderry, eight ventifacts have been found within the basalt
and flint marine gravels that underlie the sand dunes and are exposed at the base of inter-dune depressions (C797362 and C802363). All the ventifacts are basalt stones, no
facetted flint pebbles were found. In addition to the ventifacts, a large number of basalt stones that resemble partially formed ventifacts occur; these possess distinct facets but the
bounding ridges are rounded rather than sharp. The eight ventifacts are shown in Plate 10B; their/? axes range from 7.5cm to 12.8cm. They are all classed as ridged ventifacts, having
one or two well-developed facets rein-' and ^zweikanter') separated by a sharp and
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prominent ridge. Two o\ the eight display a third facet but these are less prominent and the
bounding ridges are rounded, therefore no true 'dreikanter* have been found. Original
surfaces are retained by those ventifacts possessing only one facet, and these are invariably rounded to some degree. Facets do not always occupy the entire side of a stone but are sometimes confined to the upper portions. This is probably because the stone was partially buried at the time of ventifact production. AH facetted surfaces contain micro-scale pits and
grooves to greater or lesser extent. The promi__e__ce of these features is likely to reflect textural and/or mineralogical variations in the basalts. Sugden (1964) has identified ventifacts consisting of fracture facets subsequently modified by the abrasive action of wind-blown sand. Where such facets occur it can be difficult to distinguish them from facets cut entirely by abrasion. However, it seems unlikely that the Portstewart ventifacts are modified fracture facets because although fractured stones are common they do not display any evidence of modification by wind abrasion.
The age of the ventifacts is difficult to establish with certainty. It is thought unlikely that they are inherited from a Pleistocene environment because the sharp ridges between facets would have been dulled during marine transport. The gravels were probably deposited as a gravel beach c6000 years B.P. at the peak of the Holocene marine
transgression and were soon covered with dunes (Carter and Wilson 1990). The period immediately prior to dune construction is perhaps the most likely time for their
development. This would require sufficient wind-blown sand to cut the facets but not
enough for them to be buried by accumulating sand. The ventifacts could have continued to
develop as the dunes were building up around them. The depressions in which they occur
may have functioned as major dune corridors for most of the last 6000 years and have been
regularly swept by wind-blown sand. Today the depressions are sealed at both seaward and
estuary ends, the dunes and depressions are vegetated and the ventifacts were almost
entirely buried by sand and vegetation when found. While it is clear that winds of sufficient
strength to transfer beach sand to the dunes still occur today, ventifacts are not currently being produced.
Acknowledgements
I wish to thank Nigel McDowell for taking the photographs and Kilian McDaid for
drawing the diagram.
References
Bell. H. S. (1940) Armored mud balls: their origin, properties and role in sedimentation. J. Geol. 48: 1-31. Calkin, P. E. & Rutford, R. H. (1974) The sand dunes of Victoria valley, Antarctica. Geogr. Rev. 64: 189-216.
Carter, R. W. G. & Wilson, P. (1990) Portstewart Strand and the Bann Estuary. In Wilson, P. (ed) North Co. Antrim and Londonderry: 18-23. Field Guide 13, Irish Association for Quaternary Studies, Dublin.
Dickas, A.B. & Lunking, W. (1968) The origin and destruction of armored mud balls in a fresh-water lacustrine
environment. Lake Superior. J. Sediment. Petrol 38: 1366-1370.
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Eytes, N. & McCabe, A.M. (1989) The Late Devensian ?22,000 BP) Irish Sea basin: the sedimentary record of a
