Orkney's Unique Geology: From Ancient Rocks to Glacial Landscapes

Orkney's captivating landscapes – dramatic cliffs, rolling hills, ancient monuments – are all shaped by an extraordinary geological history stretching back hundreds of millions of years. From the remnants of ancient mountains and vast Devonian lakes to the sculpting power of Ice Age glaciers and the relentless erosion of the Atlantic, Orkney's rocks tell a compelling story. Understanding this geological foundation enhances any visit, revealing why the islands look the way they do and how geology has influenced everything from Neolithic settlement to modern life. Join us on a journey through Orkney's deep time.

The Ancient Foundations

Beneath the dominant sandstone lies Orkney's basement rock, part of the ancient Caledonian mountain chain formed during continental collisions around 470-500 million years ago. These metamorphic rocks (granite, gneiss, schists) are mostly hidden but occasionally outcrop, notably the Stromness Granite visible at Yesnaby and on Graemsay. These hard, ancient rocks formed the bedrock upon which later sediments were deposited.

The Age of Old Red Sandstone: Life in Lake Orcadie

The vast majority (around 80%) of Orkney's surface geology is composed of Old Red Sandstone (ORS), deposited during the Devonian period (approx. 419-359 million years ago). At this time, Orkney lay south of the equator within a large rift valley system containing the vast freshwater Lake Orcadie.

The lake's environment fluctuated dramatically:

• Wet Periods: Deep water conditions led to the deposition of fine silts and muds, forming laminated flagstones like the Stromness Flags. These layers preserve incredible fossils, particularly fish like the armoured *Pterichthyodes* and the lungfish *Dipterus*, offering a snapshot of life in the ancient lake.
• Dry Periods: The lake sometimes dried out, allowing wind-blown sand dunes to form, now preserved as the cross-bedded Eday Sandstone. Mudcracks and fossilised raindrop impressions found in some layers testify to these arid intervals.
• Flooding Events: Coarser conglomerates indicate periods of powerful rivers flowing into the lake, carrying larger sediments.

These ORS rocks, easily quarried and split, provided the building material for Orkney's famous Neolithic monuments, including Skara Brae, Maeshowe, and the Stones of Stenness and Brodgar.

Fiery Interludes: Orkney's Volcanic Past

During the Devonian period, the Orcadian Basin also experienced volcanic activity associated with the crustal stretching that formed the lake basin:

• Hoy Lava Flows: The upper layers of Hoy's hills include thick sequences of basaltic lava, evidence of significant eruptions around 390 million years ago.
• Volcanic Vents: Remnants of ancient volcanic vents (maar-type craters) can be found in areas like Deerness and South Ronaldsay.
• Dykes: Later intrusions of igneous rock, like the camptonite dykes found cutting through older sandstone in West Mainland, indicate subsequent periods of minor volcanic activity.

While not as dominant as the sandstone, these volcanic rocks contribute to the varied geology and topography, particularly on Hoy.

Sculpted by Ice: The Glacial Legacy

Much more recently, Orkney's landscape was profoundly shaped by repeated Ice Age glaciations over the last 2.6 million years. During the Last Glacial Maximum (around 26,000-19,000 years ago), Orkney was covered by thick ice sheets flowing mainly from the Scottish mainland.

• Erosion: Glaciers scoured the bedrock, smoothing hills, deepening valleys into characteristic U-shapes (like Rackwick Glen on Hoy), and carving out basins that now hold lochs like Harray and Stenness. The vast Scapa Flow is also thought to be a glacially overdeepened basin.
• Deposition: As the ice melted, it left behind thick deposits of boulder clay (glacial till), containing rocks transported from Caithness and further afield (erratics). Meltwater rivers deposited sands and gravels in features like the Evie terraces.
• Sea Level Changes: The weight of the ice depressed the land, and subsequent melting caused sea levels to rise. The interplay between land rebound (isostasy) and sea-level change created features like submerged forests and raised beaches visible around Orkney's coasts.

Coastal Erosion: Nature's Sculpting Tool

Today, the most dramatic geological force shaping Orkney is coastal erosion by the powerful Atlantic waves. This relentless process creates iconic features:

• Cliffs: Vertical cliffs, particularly on the exposed west coast (e.g., Hoy, Yesnaby, Marwick Head), are formed as waves undercut the sandstone, causing collapses. St John's Head on Hoy is one of the highest vertical sea cliffs in the UK.
• Sea Stacks: Isolated pillars of rock like the Old Man of Hoy or Yesnaby Castle are formed when headlands are eroded, arches collapse, or more resistant sections of rock are left standing as the surrounding cliff retreats. Their formation is often guided by faults and joints in the sandstone.
• Geos: Narrow, steep-sided inlets carved out along lines of weakness (faults or joints) by wave action.
• Gloups & Natural Arches: Formed by the collapse of sea cave roofs (gloups) or the erosion through headlands (arches), like those seen at Yesnaby.

The type of sandstone influences the features: the flagstones tend to form sheer cliffs, while the softer Eday Sandstone erodes more easily into bays.

Notable Geological Sites to Visit

• Yesnaby Coast: Offers textbook examples of coastal erosion features (stacks, geos, arches), easily accessible via coastal path. Look for sedimentary structures in the flagstones.
• Island of Hoy: See dramatic cliffs, the Old Man sea stack, U-shaped glacial valleys (Rackwick), and evidence of volcanic rocks. Requires ferry travel.
• Stromness Flagstone Quarries: While mostly disused, areas around Stromness reveal the layered nature of these fossil-rich flagstones used historically for building and paving.
• Brough of Birsay: Observe coastal erosion in action and see different rock types exposed on the tidal island.

To find relevant videos, search YouTube for "Orkney geology Yesnaby" or "Old Man of Hoy formation".

Frequently Asked Questions

What type of rock is Orkney mostly made of?

Orkney is predominantly made of Old Red Sandstone, deposited during the Devonian period around 400 million years ago.

Are there fossils in Orkney?

Yes, the Stromness Flags are famous for well-preserved Devonian fossil fish. Fossils can be seen in situ at places like Yesnaby or in museum collections.

How was the Old Man of Hoy formed?

It's a sea stack formed by coastal erosion acting on the Old Red Sandstone cliffs of Hoy. The sea exploited weaknesses (joints/faults) in the rock, isolating the pillar from the mainland cliff.

Was Orkney covered by ice during the Ice Age?

Yes, Orkney was covered by thick ice sheets multiple times during the Pleistocene Ice Ages, which significantly shaped its valleys, lochs, and coastline.

Orkney's geology is a dramatic narrative written in stone, spanning vast geological timescales. From the sediments of an ancient lake to the scars of glaciers and the ongoing sculpting by the sea, understanding this foundation deepens the appreciation of Orkney's unique landscapes and its rich human history. Consider Orkney accommodation near key geological sites like Yesnaby or Hoy for immersive exploration.