Country diary: Miniature ‘oil' slicks in lofty locations

The Guardian

19-05-2025

When you move across the country, as we did a few months ago, you are changing not only a house but the landscape that comes with it. Suddenly, far-distant lochs and mountains known only from maps swing into view; unknown summits shimmer invitingly in the heat haze of this unlikely sweltering spring.
Recently, restless to explore, we scampered up the slopes of Meall nan Tarmachan, a Munro that rises high above Lochan na Lairige – high and hemmed in by the grand curving buttresses of the Lawers Dam – and looked out across the many rippling ridgelines, bare hill upon hill upon hill.
Every ascent comes as a surprise, a blowing open of the mind. But the real revelations were found on the lower slopes, which were starred with wood anemones and violets, and lit by the tiny pink lanterns of the blaeberries, just coming into fruit – long before I've been used to seeing them in Orkney. The divots were wriggling with tadpoles, shipwrecked on the shores of their shrinking pools, not long for this world.
In places, the track itself took on a shimmering appearance: the swirling, polychrome sheen of the petrol station, an opaline ooze that seeped over the ground like an chemical spill. Despite appearances, this 'bog oil' is not an oil at all but a bacterial film, the iron- and manganese-loving Leptothrix discophora, which slowly builds a delicate sheet of oxidised metal across the surface of still water.
It can be identified quite easily by touching it gently with a stick. If a suspected oil spill shatters into tiny platelets, it is bacterial in origin, not industrial, and usually indicates the presence of bog iron nearby – pea-sized metallic nodules once forged by Norsemen to make the rivets on their Viking longboats.
Leptothrix colonies thrive when water levels fall, raising the concentration of dissolved metals in standing water. And so now, weeks into our heatwave, as the long grass wilts under a hot sun and the puddles simmer away into nothing, the hillside glistens with metal-eating micro-organisms, glittering with the disco lights of a bacterial bloom.
Under the Changing Skies: The Best of the Guardian's Country Diary, 2018-2024 is published by Guardian Faber; order at guardianbookshop.com and get a 15% discount