Upper Till resting on breccia dominated by ultrabasic clasts
Beds of reworked peat in the breccia
The upper part of the peat lying below breccia and till
The sediment sequence at the rear of the geo at Fugla Ness, with the storm beach in the foreground
Striae on North Roe, after Chapelhowe (1965)
Update: The Fugla Ness peat has been analysed recently to compare its sulphur composition with that of recent peats (Bottrell and Coulson, 2003). They conclude that "Diagenetic alteration of organic sulphur isotopic compositions in pure peats appears to be minimal, and thus organic sulphur in peat sequences may record changes in isotopic compositions of atmospheric sulphur inputs through time."
Significance: One of only a handful of sites in Scotland where peat is preserved that dates from an interglacial period. The peat preserves a pollen and fossil record that demonstrates a former tree cover in Shetland. The peat lies between two tills and is covered by preglacial slope deposits. The sequence probably represents changing terrestrial environments over the last 150 thousand years.
Fugla Ness [HU 312913] lies at the northwest corner of North Roe. The local ultrabasic and metamorphic rocks have been ice moulded to leave a terrain of low hills and shallow rock basins. The Fugla Ness sediments sit within one of these basins and have been exposed by coastal erosion. Many striae have been recorded between the cliffs of the Beorgs of Uyea and the north coast, trending NNW-NW.
The uppermost deposit at the site is the Upper Till. It shows many features typical of glacial lodgement till, including erratics of Ronas Granite, striated clasts floating in a matrix of sand and grit and a clast fabric that indicates transport from the SE. Similar tills deposits are exposed on the island of Uyea. The Upper Till probably represents the last period of ice cover on Shetland, the Late Devensian.
Beneath the till is a clast- and matrix-supported layer comprising mainly debris from the local utrabasic rocks. The angular clasts, the down slope fabric and the locally-derived debris indicates that this is a periglacial slope deposit. The deposits formed during a period of intense frost activity when the thaw of frozen ground allowed down slope sludging of debris, a process known as solifluction. This breccia includes material reworked from the underlying peat.
The Fugla Ness Peat is up to 100 cm thick. The peat is compacted and thin lenses of sand and grit are confined to its upper part. The base of the peat is pebbly and rests on a thin layer of silt, probably deposited in shallow pool. A striking feature of the Peat is the abundance of wood, mainly the roots of Scots Pine.
Fugla Ness Peat, with roots and other woody debris
The peat contains pollen and a range of other plant remains. Important work by Birks and Ransom (1969) established that the lower part of the Peat accumulated when a heathland vegetation occupied the surroundings of the basin, with some pine and birch woodland, including, the presence of warmth-loving holly. The basin itself may have been a wetland, as pollen of wetland and pond species is well represented. Later the pine and birch woodland increased but the truncation of the peat by erosion means that the upper part of the vegetation sequence is largely lost. Analysis of pollen from the overlying breccia by Whittington (Hall et al, 2002) showed increased levels of grasses, suggesting a cooling of climate after the development of woodland and prior the deposition of the breccia.
There is no doubt that the Fugla Ness Peat represents an interglacial period but which one of the many interglacial phases of the Pleistocene is uncertain. Pollen evidence, notably the presence of pollen of fir, Daboecia cantabrica and Erica mackiana - the latter two, illustrated below, are today native to north-west Spain, south-west France, and western Ireland - suggests correlation with penultimate or Gortian/Hoxnian interglacial deposits in Ireland.
The warmth of the interglacial suggests however correlation with the last interglacial 125 thousand years ago. This is consistent with the presence of only a single glacial till overlying the peat. The first age attribution is supported by a thermoluminescence age on sands from the overlying breccia of 290±25 ka but the second is supported by a Uranium-Series of 110+40-30 ka on the peat itself (Hall et al, 2002).
The rich maritime heath and local woodland that developed at Fugla Ness shows the establishment of a mild, oceanic climate, perhaps warmer than today and with lower wind speeds. The changes in vegetation in the upper part of the peat and the organic lenses reworked in to the overlying breccia indicate a increase in first and cooling temperature that terminated in the onset of cold conditions.
Excavations using a mechanical excavator confirmed that the Peat rests on another till layer and on ultrabasic bedrock. The Lower Till is grey and dominated by locally-derived clasts but includes some granite erratics. It also shows as strong NW-SE clast fabric. The Lower Till is the oldest glacial deposit known on Shetland. It is significant as it suggest that the rock basin in which the sediment sequence sits was already in existence before the last glacial maximum.
No direct marine influence has been detected at the site, either in the pollen record or in the sediment sequence. This suggests that the coastline lay some distance to the west as the sediments accumulated.