To get to the tidal island known as Brough of Birsay in Orkney, to see the old monastic ruins and the birds, you have to cross a causeway (HY 242 284). At low tide a concrete pathway zig-zags across the rocks allowing access for a limited time. And what rocks they are! Row after row of sloping strata reveal their jagged edges. The flat bedding planes of this Upper Stromness Flagstone show preserved ripple marks from the time when the rocks were formed on the bed of the enormous freshwater Lake Orcadie that existed nearly 400 million years ago in the Devonian Period. The lake extended from the Moray Firth, across Caithness to Orkney, Shetland and beyond to the Norwegian coast. The consolidated sediments underpin much of the Orkney islands and reveal themselves along the shorelines. Now, deep pools with seaweed and seashore creatures occupy the valleys between the broken and weathered rock layers, and wait to be explored on sunnier days at lower tides. Heaps of multicoloured seaweed drifts ashore here; and between the outcropping bedrock lie dark grey and rusty coloured beach stones and boulders, with sand made of crushed seashells.
Jessica's Nature Blog 
Very beautiful pictures!😊 Like the shape on the stone in picture 2.
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The ripples in the rocks are incredible.
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Rocks are so colorful and fascinating to me. I love the orange and slate mix of the ones you’ve photographed in Birsay Bay.
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Thank you, John. Sometimes the rocks have strong almost geometric patterns.
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I am always intrigued that features of the seabed and seashore that we can see everyday, like the sand ripples, have been so beautifully preserved in sedimentary rocks from hundreds of millions of years ago.
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Thank you, Bespoke Traveller. I suspect that the orange rocks may be weathered examples of the grey ones. In Dorset there are blue-grey rocks that turn brown when exposed to the elements. Something to do with iron compounds in the matrix oxidising. Either that or there are alternating layers of grey and orange rocks at this Orkney site.
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Its incredible to think those temporary patterns have achieved a sort of permanence.
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It is amazing, Emma, because the circumstances for preservation have to be just right – and yet it happens over and over again, not only in these Devonian lake bed sediments in Orkney but elsewhere in the UK and globally.
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