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.