Country diary: treasures that were once beneath the Cambrian sea | Environment

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.

“Stromatolites,” a geologist told me, when I showed him photos of these round rocks, beautifully layered in shades of grey: fossils created around 500m years ago by cyanobacteria (also known as “blue-green algae”). This was a flat, watery place then, before a continental collision pushed up the mountains we see today. In the shallows of the Cambrian period seas, colonies of these photosynthetic aquatic organisms built columns from layers of sediments and calcium carbonate. The fossils lie on the Eilean Dubh Formation, a geologic stratum often marked by coral and shell fossils, the remnants of other creatures who also inhabited those ancient seas.

4000 - Country diary: treasures that were once beneath the Cambrian sea | Environment

Fossilised stromatolites. Photograph: Robin Patten

Although living stromatolites can still be found in places such as Australia and South America, they are now rare, due to changing conditions over the eras; the fossilised version might be easier to find, and of course is the only version in Assynt.

The three stromatolites I’ve found here are chunks of columns, broken into partial rings and at least a foot across, larger than normally found in the Eilean Dubh dolostone, according to that geologist. Despite their size, they are easy to miss, blending into the grey rocks strewn around. Yet once spotted they are hard to overlook, and worth seeking out at any season. Today, the intricately textured fossils curl gracefully around moss and sedge. When I return in summer I will find miniature rock gardens, a rich array of colourful leaves and blossoms nestled within the stones’ spherical centres in displays that look more like art than fossil.