The 110 thousand year old main peat band
Layers of organic sand and gravel
Bleached sands and stones with weathering rinds
Significance: With Fugla Ness, one of only two sites on Shetland that preserve a detailed record of environments on the islands before the passage of the last ice sheet. The organic deposits at Sel Aye represent vegetation covers ranging from grasses to a rich herb flora that have responded to past changes in climate and site conditions. The organic materials are inter-bedded with periglacial slope deposits and overlain by glacial till. Sel Ayre provides a snapshot of conditions on Walls around 110 thousand years ago.
Sel Ayre (HU 176540) lies at the western extremity of the Walls peninsula. The sediments infill a gulley and have been exposed by cliff retreat. The site lies above steep cliffs and access may require the cutting of steps or the use of a rope.
The upper till is up to 4 m thick and carries erratics of Devonian sandstone, many of them facetted and striated. The presence of metamorphic erratics and a strong clast fabric implies ice flow from the southeast, equivalent to the pattern of flow of the last ice sheet across Walls.
Excavation beneath the till has revealed a sequence of crudely bedded sand and blocky gravel forming the Upper Breccia. Some clasts have weathering rinds and sand lenses show iron-staining, suggestive the start of soil formation. The material accumulated in the gulley during phases of strong frost weathering.
The Sel Ayre Organic Sands and Gravels are up to 3 m thick. The upper part of the sequence comprises beds of sand and rubble, with thin lenses of organic material. Bleached sands and weathered clasts occur in this sequence and appear to relate to the stripping of soils from around the site.
A layer of peat, up to 50 cm thick but often poorly exposed and requiring excavation, occurs along the mid-line of the gulley. It is highly compressed and passes towards the gulley margins in to inter-bedded peat layers and organic sands. It rests on a thin layer of pale green mud.
The lower sequence of bedded sands and angular blocks also appears to have a periglacial origin. It is an openwork rubble with a strong downslope fabric, a typical scree deposit.
Fill of the lower part of the gulley, a scree deposit that accumulated soon after the last interglacial
The sediments at Sel Ayre probably relate to changing environments around 110 ka (Hall et al, 2002). The peat layer contains a pollen assemblage (Birks and Peglar, 1979) that it is distinct from that at the interglacial site at Fugla Ness. The pollen diagram by Graeme Whittington shows that it is dominated by grasses and heaths with much more limited tree pollen, probably far-travelled. The Balkan heath, Bruckenthalia, is present and indicates correlation with theBrÝrup and Odderade Interstadials of NW Europe (Oxygen Isotope Stages 5c and 5a). The evidence of increased slope activity and the dominance of grass pollen in the overlying beds (pollen diagram 2) indicates a deterioration of climate towards periglacial conditions.