Geology matters 2 – Orkney rocks!

By Dr Martha Johnson

The majority of Orkney’s rocks are sedimentary, specifically clastic sedimentary rocks.  These are formed from the compression of particles –  clasts – of eroded rock materials.

Orkney’s sedimentary rocks were formed during the Devonian period (415-360 million years ago), when what is now Orkney was the floor of a large inland sea, Lake Orcadie. 

During the millions of years of the Devonian, the region of the inland sea underwent a variety of climatic changes and the rock formed from the sea’s sediments reflect that variety. 

Most of the deposition of silt and sand size particles onto the lake floor built up thousands of metres of sediment in horizontal layers. 

However, some sandstones are cross-bedded indicating the deposit of their sand on land by wind during drier climatic episodes.

Over the intervening millions of years, the deeply buried sediments were compressed into a variety of sedimentary rocks, from very fine-grained shales and mudstones to medium to course-grained sandstones. 

The bedrock of the Ness of Brodgar isthmus and the majority of Orkney’s West Mainland are members of the Lower Stromness Flagstone (LSF) and Upper Stromness Flagstone (USF) formations.

These are composed series of grey and tan rocks ranging from mudstones to sandstones deposited in a lacustrine environment.   

Beyond the extensive suite of sedimentary rocks, Orkney offers a variety of other rocks (and minerals).

Across the West Mainland, there are several outcrops of igneous rock, known locally as Stromness Granite, formed deep within the earth’s crust over 500 million years ago.

A much younger (300 to 250 million years old) igneous rock, camptonite, is found in thin linear intrusions throughout the West Mainland. 

Neither of these igneous rocks outcrop at the Ness of Brodgar, though there are three camptonite intrusions a mile to the south-east of the excavation site.  

Further afield, there are basalt lava flows in the parish of Deerness and on the islands of Hoy and Shapinsay.  There is also one rhyolite flow at Bay of Navershaw, near Stromness.

One geologic process has added rock to the archipelago in the past 15,000 years, glaciation.

During the last glacial advance of the Pleistocene, ice flowed from north-east Scotland north-west over what is now the North Sea across Orkney and out into the Atlantic. Trapped and transported beneath the ice were rocks and ground rock debris. 

When the ice reached the south-east faces of the islands some of this glacial till was deposited on the cliffs and beaches. 

The till contained rocks unique to from North-East Scotland and flint nodules from the floor of the North Sea.  Much of Orkney was coated with a layer of glacial till – from one to many centimetres in depth.

Answers from the Foreign Stone

Answering the two research questions mentioned in the previous article, provided insight into the distance and direction the Foreign Stone rock recovered at the Ness was transported. 

It also allowed the analysis of the occurrence and distribution of various Foreign Stone within the site.

Most importantly, identifying the Foreign Stone, and its possible source, demonstrated the geologic knowledge the Neolithic Orcadians possessed and choices they made with respect to rock.

If the 2,988 Foreign Stone rocks had been gathered from the Ness of Brodgar peninsula, then they would have all been sedimentary rocks.

That was not the case.

Of the 669 igneous rocks found on side, 334 were a type (basalt, basalt porphyry, granite or rhyolite) with no outcrops near the site. 

How did they get there? People clearly brought them. 

The situation with sandstone was similar. Owing to the volume of sandstone Foreign Stone finds, colour was used to create sub-groupings (grey-tan, buff-white and red-pink) for them.

Though there are no outcrops of red-pink sandstone on the Ness of Brodgar, of the 1,761 sandstone Foreign Stone finds, 1,032 (58.6 per cent) of these finds were red-pink in colour.

The above comparisons suggest different uses for different rocks.

The Neolithic Orcadians appear to have recognised unique and desirable properties in different rock and made rock selections based, in part, on their properties not on just on nearby access to a rock outcrop.

Rocks were not all “just rocks” to them.

To be continued…

Further Reading

• Rock matters: A geological basis for understanding the rock at the Ness of Brodgar. Dr Martha Johnson (2019)