Island of Hoy, Orkney – an excursion

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From: Fannin, Nigel G.T. Edinburgh Geological Society field excursion Orkney May 1991 (unpublished)

Locality 1 Rackwick (ND 996 198) to Old Man of Hoy (HY 176 008)

The island of Hoy is composed largely of Upper Upper Old Red Sandstone with an area of Middle Old Red Sandstone flagstones at the northern end of the island. The end of Middle Upper Old Red Sandstone times was marked by gentle folding and faulting and a period of significant erosion which resulted in down cutting of the Middle Old Red Sandstone so that in places the Upper ORS sediments rest directly on Stromness Flagstones.

Volcanic activity, first seen as a series of thin ash beds in the Stromness Flagstones, increased throughout the Middle Old Red Sandstone and the irregular surface of the Upper Upper Old Red Sandstone unconformity is blanketed by ashy sediments and a partial cover of basalt up to 90 m thick.

The overlying, almost horizontal, Hoy Sandstones are at least 1000m thick. They comprise red and yellow medium grained, massive and cross-bedded sandstones with thin grey or greenish marly partings. Cross sets are rarely more than one metre thick. Slumping and convoluted bedding is common in some places with intraformational conglomerates at the base of some cross-sets. The sandstones were probably laid down in a system of braided rivers.

From Rackwick a path leads westward via the south shoulder of Moor Fea to the cliff top opposite the Old Man of Hoy. A difficult path leads from the cliff top to the foot of the Old man. As this includes an exposed traverse it should be undertaken only in dry weather by the sure-footed and, with proper footwear. The exposures at the foot of the Old Man, include a base of gently inclined Upper Stromness Flags, here overlain directly by lava, which is, in turn, overlain by the reddish Hoy Sandstone. The lateral variations in thickness of the lava are well seen from here. Mud filled dessication cracks and mudstone chips are common along bedding planes at the base of the Old Man. Near the top of the cliff the sandstone shows large-scale, mainly planar, cross-bedding, suggesting deposition in wind-blown dunes.

Locality 2 Rackwick (ND 198 990) to Too of the Head (ND 190 988)

In Rackwick Bay, looking south east, towards Craig Gate (ND 206 977) and across the Bring fault which forms the southern edge of South Burn valley, the massive and cross bedded red and yellow sandstones form spectacular cliffs. The fault downthrows the beds to the northwest by approximately 1000m.

At the western end of the bay (ND 196 990) soft yellow fine to medium grained cross bedded sandstones with thin marls and flags and rare pebbly lenses underlie the Upper Old Red Sandstone ashes. These beds are interpreted as Lower Eday Sandstones, but their age has not been proven.

Westwards, towards Too of the Head, these sandstones are seen to be overlain by ashes of Upper Old Red Sandstone age. Here the tuffaceous sediments appear to blanket the existing topography as an air fall deposit. Elsewhere they appear to have been deposited in, or redistributed by, water. The sediments are dominantly brown or reddish tuffaceous sandstones with blocks and pebbles of basalt. The ash layer is up to 15m thick but varies considerably with the local topography.

Overlying the ashes, around Too of the Head (ND 190 988), columnar basalts appear to fill an existing valley and disappear abruptly to the west. The lava is an olivine basalt with porphyritic crystals of feldspar and olivine in a ground mass of iron ores, augite and plagioclase and is up to 60 m thick with a purple slaggy top.

Locality 3 Rackwick Bay (ND 200 988)

A well developed storm beach forms the western end of Rackwick Bay with finer sediments forming the eastern end of the bay.

Locality 4 Moss of Whitestanes (ND 215 995)

Hummocky moraine is well developed along the valley of the South Burn and towards Rackwick Bay mounds of fluvio-glacial outwash gravel and a conspicuous terrace with a flat kettled surface are seen.

Locality 5 Dwarfie Stane (HY 244 005)

A unique rock-cut Neolithic tomb has been cut out of a single block of cross bedded Hoy Sandstone measuring 28 ft x 14 ft x 6 ft.

Locality 6 Mo Ness (HY 246 040)

The most southerly outcrop of the Sandwick Fish Bed is poorly exposed around Mo Ness and at the northern end of the Bay of Creekland (HY 237 048). Time and tide permitting these can be examined.

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