The Great Orcadian Lake
In the Island of Westray
WEBINAR
10th April 2024 19:00 GMT
by David Leather
Abstract
Westray is the most north-westerly of the Orkney isles and much of the bedrock is composed of a thick series of cyclic lacustrine flagstones of Middle Devonian age. Cycles average 10 to 12 m in thickness and each may be subdivided into permanent lake, transition zone, playa/floodplain, and fluviatile sections, as the lake regularly dried up and returned, following the 100,000-year Milankovitch cycle of orbital forcing. Lake bed intervals contain well-preserved fossil fish known for almost 200 years to collectors such as Hugh Miller. Here in the Great Orcadian Lake, predating fish were by far the main inhabitants, most developing armour plating that enshrouded head and shoulders, protecting the most vulnerable parts, with sizes from a few centimetres to a metre and a half in length.
Some 390 million years ago during Eifelian-Givetian time, when continents were grouped near the south pole, the Great Orcadian Lake lay in an inland basin about 16° south of the equator probably not far from the sea into which it occasionally overflowed. Some of the fossil fish proved good biozone fossils and following Uisdean Michie’s subdivision of the Rousay Beds, new geological boundaries began to emerge. The island showed an overall synclinal structure with the oldest beds in the core, just as Peach and Horne described when they visited the island in the late 1870s. Sedimentary structures such as wave and current ripples, desiccation cracks and pseudomorphs of gypsum are widespread, and faults plotted onto a 2½ inch Ordnance Survey map eventually revealed a fine flower fault structure, originating in Scapa Flow.
Biography
David Leather has been a member of YGS for over 60 years. He grew up in Blackburn where he attended Queen Elizabeth’s Grammar School and went on to read geology at Keele University under Professor Cope. In the mid-1960s he spent two years teaching in the highlands of western Kenya where he and his wife travelled widely. He became head of Geology at Sir Titus Salt School, Shipley, where many of his students took the subject to university level and beyond. On early retirement he wrote a series of guided walk books on the Yorkshire Dales that include aspects of geology and natural history.
For the last quarter of a century, based in a remote cottage, all his summers have been occupied investigating the geologist’s paradise of Westray. Here flagstones, fossil fish, faults and sedimentary cycles galore are newly exposed each spring along the whole of the island’s long indented coastline.
With Belgian PhD student, David De Vleeschouwer, he recorded the orientation of over 500 wave ripples which show new details of the palaeoclimate south of the Devonian equator. With guidance from John Flett Brown, he plotted 120 faults around the coasts of Westray and Papa Westray that proved a flower fault system originating in Scapa Flow. In 2009, his exhibition at the Westray Heritage Centre won the ENI Geological Challenge Award with a presentation at Burlington House and the exhibition was repeated the following year. A revised edition of his book Westray Flagstone was published in 2022.