The 50-acre prehistoric forest, on the west side of Otterswick Bay, Sanday, was first noted in the mid-19th century, after a storm exposed moss with tree roots sticking out of the sand at low tide.
Then, Sanday historian Walter Traill Dennison wrote: “It is an melancholy sight to look into the open grave of what had one time been an umbrageous forest, blooming in all the sylvan beauty of stately trunk, spreading bough, and green; leaves; where beasts roamed and fair birds sang.”
The forest was first investigated in March 1850, and appeared on a hydrographic chart of the North Ronaldsay Firth, surveyed in 1847-48. It records the site of the “Submarine Forest” between Lamaness Skerry and Helliehow.
In 1867, in his History of Orkney, the Rev George Barry wrote: “There is a general and strong tradition that the harbour of Otterswick in Sanday was once a forest, which was destroyed by inundation.”
He added: “Deerness is also reported to have been anciently a considerable forest, which deluge overwhelmed”.
Traill-Dennison obtained a sample from one of the Otterswick trees, in March 1890, and, in a paper written in 1893, was already making references to the “sinking process that is going on at the present moment”.
But although various anecdotal references were made to the Sanday forest, until now no one had actually recorded exactly where it lay.
Geomorphologist Alistair Rennie discovered the remains while studying the effect of rising sea levels in Sanday. Funded by Scottish Natural Heritage and Glasgow University , the investigation became aware of a number of local traditions that referred to a sunken forest in Otterswick. An 1847 map produced by one Commander Becker also had a submerged forest marked on it.
So knowing roughly where to look, test digs were made in the bay at low tide.
Hope was beginning to fade after five unsuccessful attempts, but the sixth pit revealed a layer of peat containing fragments of tree.
Lying beneath 75cm of sand and 10-15cm of shell fragments, the excavation revealed a 10-15cm layer of peat along with tree branches. Beneath the layer of vegetation was a thin layer of till on top of weathered bedrock.
Twelve tree samples were removed and transferred to Glasgow for identification. These turned out to be salix, a willow tree, and standing about nine feet tall, were once fairly substantial.
Carbon dating revealed the trees to be 6,500 years old – in other words the forest had flourished around 4,500BC, about 1,300 years before the first settlement at Skara Brae.
The discovered has shown that the sea level has risen by three metres over the subsequent centuries – an estimated rate of 1-3mm a year.