The East Mainland
GEOLOGY

The geology of Deerness, Holm and St Andrews comprises the Rousay Flags and Eday Beds. These rock series, named after the type locality where they are best displayed, are of Old Red Sandstone (Devonian) Age and around 370 million years old.

In Old Red Sandstone times, Orkney was part of a tropical continent with a hot and humid climate. Surrounded by mountains, the Orcadian Lake extended at times from Moray to Shetland.

Response to earth movements up and down caused the lake to extend, retreat or dry up. Deeper water phases are shown by fine-grained mudstones or flags, shallowing by coarser-grained sandstones, often exhibiting ripple marks or cross-bedding, and drying out by sun-cracks and algal coatings.

Sediments were carried into the Basin by streams and rivers flowing off the nearby mountain ranges. Rainfall was probably of a “flash-flood” nature and the river channels which are left fossilised are the result of many changes of direction of deposition.

The underlying older Rousay Flags found to the west in Holm and St Andrews are overlain by the next younger set – the Eday Beds – in a trough to the east called the Deerness Syncline.

In the west, the East Scapa Fault parallels the Holm shoreline – the same fault as marks the eastern boundary of the Kirkwall Valley. Unique to the Orkney Mainland are two Deerness localities at Muckle Castle (HY 563032) and Point of Ayre (HY 591038).

Muckle Castle or Stack is a volcanic plug, like Edinburgh Castle Rock, and appears as green amorphous material, very distinct from the wall-bedded Eday Flags, which it intrudes. This may have been the vent of a volcano which spewed out the associated lava and ashbeds.

Fifty metres east of Muckle Castle, and only accessible at low tide, is the enrolled slab. This unique feature is a “swiss-roll” of flagstone which has been rolled over while it had a plasticine consistency, then fossilised as an example of the minor earth movements that were going on at the time. At the Point of Ayre is a seven metre thick basalt lava flow. At its top surface are relict gas bubble holes now filled with carbonate. Lower down, the lava weathers in an “onion-skin’6 or spheroidal pattern, an unusual feature in Orkney.

 

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