9.—On the Ecology and Sedimentation of the Cardium Shellsands and Transgressive Shellbanks of Traigh Mhor, Island of Barra, Outer Hebrides

Abstract

Synopsis Holocene carbonates rest unconformably on Lewisian gneiss in an almost land-locked bay sheltered from Atlantic waves by sand dunes: frosts are rare and cockles, the dominant species, may reach 16 years of age. Five intertidal zones are recognised: algal mat, Arenicola sand, Cardium shell sand, Lanice sand, Ensis sand. Transgressive shellbanks composed solely of cockles and linguoid in form, are advancing over the Arenicola sand towards the shore, earlier seaward positions being indicated by hard-packed areas of shell pavement. Comparison of 1973 positions with those on aerial photographs taken in 1965 demonstrates a maximum transgression of 150 m over the past eight years, but in 1948 banks were smaller and most material was onshore as a chenier. Different age-groups are dominant in different parts of the beach: young cockles high on the shore; older, partially epifaunal individuals towards low water. In any one age-group shell height is inversely related to distance from LWM. By analysing the age and growth of dead shells from the banks it is possible to estimate where they originated, and thus how much post-mortem transport they have suffered. In the centre of the bay banks receive 90 per cent of their shells from seaward, the maximum distance travelled being 400 m. During westerly gales adult cockles living high on the shore are blown seawards, though young forms escape by speedier burrowing. This, together with the concentration of oystercatcher predation on small individuals from the more emergent parts of the bay, probably explains why older animals are more abundant on the lower reaches of the shore. Feeding oystercatchers leave many conjoined valves of Cardium on the sediment surface which roll in easily with the tide and replenish the shellbanks. The valves are rarely damaged, though gulls shatter shells by dropping them onto hard areas of pavement. In calm conditions shoreward flotation of both fragments and single valves is important: banks are high and narrow, and accrete by avalanche co-sets of haphazardly arranged whole valves. In stormy conditions much comminuted shell sand is blown onto the beach from the dunes: banks are more spread out, and the shell fabric locally becomes vertically imbricate in response to directly opposed wave systems generated by refraction around the banks.