Country diary: treasures that were once beneath the Cambrian sea

Assynt, Sutherland, Highlands: The stromatolite fossils lie on the Eilean Dubh Formation, a geologic stratum often marked by coral and shell fossils

As I climb up from the green-brown valley near Inchnadamph, the early spring countryside changes character. Snow patches appear and soon become abundant, then all seems white as the mountains' snow-blanketed slopes merge into silver-grey clouds. On this blustery day, when sleet and rain slash across the landscape and wind snatches at all things, it's hard to believe the Highlands were ever anything but a cold, damp, mountainous place. But the curious circular rocks embedded in the foothills are evidence that the earth beneath my feet once lay under shallow seas in a considerably warmer climate.

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