Walking the route round the shore took me through the street to the end of the flagstones. I was walking on different parts of Lake Orcadie with each step.

It’s a classic walk, through the Stromness street and out to the West Shore, with the light opening up over Hoy Sound and the hills of Hoy behind. Artist Rebecca Marr looked closely at the flagstones beneath her feet and found they were carrying the imprints of their formation.

Once they were sediments on the bed of a great freshwater lake, 400 million years ago. It was set in a hot mid-continental desert and sometimes the shallows would dry and bake and crack in the sun. Sometimes water currents would leave ripples in the sand. The mud and the ripples are long gone but the shapes remain.

In Walking Lake Orcadie Rebecca, a photographic artist whose gaze is often on the natural world, among clouds or plants or seaweed, focuses on flagstones as individuals, each shaped by time.

In close-up, she says, the stones form compositions, images that range from moonscapes to figures in a dance, each one “a still image made in a time of flux”.

Like photographic plates they have been exposed and now they carry a message, they communicate something about a particular place and time. They are a memory.

An introduction by archaeologist Antonia Thomas looks at the world of the ancient lake and its imprints that remain.

Violent rain showers, which may have lasted a few hours at most, have left their mark after all these years in the pitted flags. Eroded residues of crystals, which once grew in the lake’s damp sands, now survive as a fretwork skin on the top of paving slabs.

The fish and plants have long gone, but if you look hard enough, you might find one of their ghosts: a fossil, trapped forever in stone.