The Caithness Flagstones are part of a thick Middle Devonian sequence, generally considered part of the continental Old Red Sandstone facies, which crops out in northeast Scotland, including the islands of Orkney and Shetland. The succession consists of lacustrine and fluvial beds deposited in a large internal basin with no evidence of marine connections. Between 5 and 6 km thickness of light gray, red-brown and gray-green cross-bedded sandstone, greenish-gray laminated siltstone and gray-black laminite and limestone are exposed along the east and north coasts of Caithness. Many of the gray-black laminites and a few of the greenish-gray laminated siltstones exhibit numerous small-scale (50 mm plan length) linear cracks. The cracks are in fine siltstone and have been infilled by coarse siltstone. They are attributed to subaqueous shrinkage by a syneresis mechanism. About one-half of the crack-bearing beds show a persistent crack orientation in plan, the direction of which corresponds with the long axes of ripple marks. The preservation of the cracks is a reflection of the placidity of the lacustrine environment.

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