Island of Eday, Orkney – an excursion

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From: Fannin, Nigel G.T. Edinburgh Geological Society field excursion Orkney May 1991 (unpublished)

Locality 1 Fersness Bay (HY 534 338) to Sands Of Mussetter (HY 544 333)[edit]

The section provides a complete sequence of the Lower Eday Sandstone, the Eday Flags and the Middle Eday Sandstone along the western limb of the Eday Syncline. The Lower Eday Sandstone consists of a lower facies of predominantly reddish-purple trough-cross-bedded sandstone with pebbly lenses throughout and with conglomeratic channel fills approximately 30m above the base, and an upper facies of yellow sandstone with rarer pebbles and predominantly planar-cross-bedding. The yellow sandstone makes an excellent building stone which was in the past exported directly from the quarry at HY536 336 to various Scottish ports. Partly dressed freestone blocks are still to be seen in the quarry. The Eday Flags consist here of only two cycles with recognisable fish beds, though a further band of fish bearing calcreous siltstone appears at HY 530 335, some 110m above the base of the Middle Eday Sandstone facies. The latter is a deep red pebbly and locally conglomeratic fluvial sandstone, with well developed trough-cross-bedding, scour and fill structures, ripple drift and convolute bedding. A feature of this section is the presence of abundant pebbles of porphyritic and spherulitic rhyolite and scoriaceous basic lava. Good sections of boulder clay overlain by blown sand, are seen on the shore east of HY 543 334.

A fine section of till occurs both on the coast of Fersness Bay and along a deeply incised stream just west of the Sands of Mussetter. The section, described by Peach and Horne (1880) exposes at its western end some 8m of a tough red boulder clay containing erratics of predominantly local origin. The clay also contains shell fragments and rare exotic erratics. Where the stream has cut down to bedrock, sandstone surfaces bear striae with orientations generally between south-east to north-west and south to north. A large roche moutonnee near the mouth of the stream indicates a flow to the north. At the eastern end of the section, red till can be seen to overlie brown till.

The age of the till is not known with certainty although it is thought that the Shelly boulder clay of Caithness and Orkney may be of Devensian age, whilst an early Devensian age; based upon a radiocarbon date obtained from shells in the till at Mill Bay, Stronsay HY 662 255 has been suggested. The significance of the Fersness locality lies in the exposure it provides of one of the extensive till sections in Orkney.

Locality 2 Bay Of Newark (HY 554 364) to Greenan Nev (HY 550 370)[edit]

The Eday marls are well exposed along this section. They comprise several fining-upwards cycles consisting of relatively thin channel-fill sandstones (<2 m thick), overlain by thick overbank deposits (up to 15 m) of bright red sandy micaceous siltstone with sun cracks, ripple marks and small scale scours. There are some hard calcareous bands and many pale calcareous concretions. The siltstone has been bioturbated by burrowing organisms, producing circular near-vertical burrows up to 50cm long. If time permits the Upper Eday Sandstone will be examined along the shore just north of Greenan Nev.

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