The lost lakes of Britain (Part 1): The Orcadian lakes – reconstructing Scotland’s lost Devonian world

Jon Trevelyan (UK)

Simplified summary of Britain’s geological history from the Precambrian to the present day. The coloured timeline shows the major geological periods and the illustrations above represent some of the dominant landscapes and environments that characterised each interval. The red line(s) indicates the geological period(s) covered by this article.

This is the first of a three-part series exploring Britain’s lost lakes – landscapes that once existed across very different timescales, from the Devonian world of the Orcadian Basin to the fleeting ice-dammed lakes of the last Ice Age. In this opening article, we go deep into the Devonian, reconstructing the Orcadian lakes from the evidence preserved in Caithness and Orkney. Here, finely layered sediments, exceptional fossil preservation and subtle environmental clues allow us to rebuild not just the organisms that lived there, but the structure and behaviour of the lakes themselves.

The flagstones of Caithness and Orkney preserve more than fossil fish. They record a vanished landscape – Devonian lakes, layered waters and ecosystems shaped by unusual environmental conditions. By combining field evidence with modern geological understanding, it is possible to reconstruct not just the organisms that lived there, but the structure and behaviour of the lakes themselves.

A lost landscape in stone

Modern northern Scotland is a landscape shaped by ice and water – cool, wet and deeply eroded. Its lochs are modest features, scattered across peat and rock. It is not a place where one expects to find the remains of extensive, long-lived lake systems.

Yet in the flagstones of Caithness and Orkney, a very different Scotland is preserved. Split open these rocks and they reveal more than isolated fossils. Fine-grained slabs yield entire fish, often lying undisturbed, as if only recently settled on the lake floor. These are not the fragmentary remains of energetic environments, but the products of quiet water and exceptional preservation.

From such fragments, geologists have reconstructed a landscape that no longer exists. During the Devonian, around 400 million years ago, northern Scotland lay within the Orcadian Basin, a broad continental depression in which lakes developed across low-lying terrain (Fig. 1).

Fig. 1. Location of the Orcadian Basin in northern Scotland. Devonian lake sediments are widely exposed in Caithness and Orkney, where fine-grained flagstones preserve an exceptional fossil record of early fish and lacustrine environments.

These were not single, permanent bodies of water, but shifting systems shaped by sediment supply, basin structure and climate. It is within these lakes that the most distinctive features of the Orcadian system emerge.

Where and what is the Orcadian Basin?

Northern Scotland is not an obvious place to look for the remains of large lakes. Today, the landscape of Caithness and Orkney is open, windswept and shaped by glacial processes. Yet the rocks beneath record a very different setting.

During the Devonian, this region formed part of the Orcadian Basin, within which sediment accumulated over millions of years. Rather than a single lake, the basin hosted a shifting network of water bodies, controlled by sediment supply, climate and topography.

What makes these rocks remarkable is their fidelity. Split along bedding planes, they frequently reveal complete fish preserved with extraordinary detail (Figs. 2 and 4).

Fig. 2. Articulated specimen of the Devonian lobe-finned fish *Osteolepis* preserved in fine-grained lacustrine sediment. The completeness of the skeleton reflects minimal disturbance after death and gradual burial in quiet water conditions. (*Image: Ghedoghedo, Naturalis Biodiversity Center, Leiden. CC BY-SA 3.0.)*

Bodies lie flattened but intact, showing little sign of disturbance. Such preservation points directly to quiet water and unusual conditions.

Building the basin system

To repeat, the Orcadian Basin was not a static depression filled with water, but a dynamic landscape shaped by sediment transport and deposition.

Fig. 3. Schematic reconstruction of the Orcadian Basin showing sediment supply from surrounding uplands through braided river systems. Coarse deposits accumulate near basin margins, while finer sediments settle in low-lying areas where lakes develop.

Sediment was delivered from surrounding uplands by braided river systems, which redistributed material across broad areas. Near the basin margins, coarse deposits accumulated in alluvial fans and channels. Further into the basin, finer sediments settled out under lower-energy conditions.

This transition created low-lying areas in which water could collect, forming lakes that expanded and contracted over time. These lakes were therefore not isolated features, but integral to the sedimentary system.

Fig. 4. A second articulated specimen of *Osteolepis* from lacustrine deposits. The largely intact skeleton reflects deposition in quiet water conditions with limited disturbance. (*Image: Ghedoghedo, Senckenberg Museum, Frankfurt. CC BY-SA 4.0.)*

How the lakes worked: a stratified system

Quiet water alone, however, is not enough to explain the quality of preservation seen in these rocks.

These lake systems developed under a broadly seasonal, semi-arid Devonian climate, in which episodic rainfall and high evaporation rates helped sustain closed or weakly drained basins.

The key to understanding the Orcadian lakes lies in the organisation of the water column. Rather than forming a well-mixed body, the lakes were often stratified, with oxygenated surface waters overlying deeper layers in which oxygen was progressively depleted (Fig. 5).

Fig. 5. Reconstruction of the Orcadian lake showing vertical stratification. Oxygenated surface waters overlie deeper, oxygen-poor layers. This structure restricts life to the upper waters.

This separation had an important consequence. Oxygen introduced at the surface could not easily reach the deeper waters, as mixing between the layers was limited. At the same time, organic material – plankton, plant debris and the remains of organisms – continued to sink through the water column. As this material decayed, it consumed the available oxygen in the deeper layer.

Because this oxygen was not replenished, levels were progressively reduced. Once depleted, the deeper water remained in this state, creating persistently low-oxygen conditions. In effect, the lake developed a stable vertical structure in which oxygen was continually supplied to the surface, but steadily removed at depth.

Life in a restricted system

The Orcadian lakes supported a distinctive freshwater fauna dominated by fish (Fig. 6).

Fig. 6. Reconstruction of the Orcadian lake ecosystem. Fish inhabit the oxygenated upper waters, while deeper layers remain largely uninhabited.

Fish were largely confined to the oxygenated upper waters. Below the stratification boundary, the deeper waters supported little active life. In particular, the absence of scavengers is significant.

The Orcadian ecosystem was therefore structured by a sharp division between zones of activity and zones of stillness.

Death, stillness and preservation

The exceptional preservation of Orcadian fish follows directly from these conditions (Fig. 7).

Fig. 7. Sequence of events leading to fossil preservation. (A) fish live in surface waters. (B) after death, the body sinks. (C) it reaches the lake floor. (D) the remains settle undisturbed and are buried.

When a fish died and sank into deeper water, it entered an environment in which decay and disturbance were greatly reduced. Bodies could settle gently onto the lake floor without being broken apart.

Once on the bottom, remains were buried by slowly accumulating fine sediment; and burial was gradual, not catastrophic.

Seasonal rhythms in stone

One of the most striking features of the Orcadian lake deposits is their fine, regular layering. In places, these laminations occur as paired light and dark bands known as varves, each representing a single year of deposition (Fig. 8).

Fig. 8. Formation of varves in a lake environment. Seasonal variations in sediment supply produce paired light and dark layers, each representing one year of deposition. Their preservation indicates calm, undisturbed bottom waters.

In a typical cycle, lighter layers form during the summer months, when increased runoff brings coarser sediment into the lake. Darker layers accumulate more slowly during winter, when the lake surface may have been ice-covered and sediment input was minimal.

Crucially, these layers could only form in calm, undisturbed conditions. The presence of well-preserved varves therefore supports the idea that the deeper parts of these lakes were largely anoxic, with little or no bottom-dwelling life to disturb the sediment.

In effect, the rocks record not just the existence of the lake, but its seasonal pulse – year by year, layer by layer. In this respect, the Orcadian lakes offer a useful contrast to the much younger Ice Age lakes explored later in this series, where similar processes operated over far shorter timescales.

Hugh Miller and the first reconstruction

Fig. 9. Hugh Miller (1802-1856), Scottish geologist and author of *The Old Red Sandstone*. A self-taught stonemason, he recognised that the Orcadian fossils recorded ancient freshwater environments. (*Image: Public domain. Wikimedia Commons.)*

The importance of the Orcadian rocks was first recognised by Hugh Miller.

Fig. 10. The author’s Everyman copy of The Old Red Sandstone, dated 1909.

Modern geology now interprets many of these fish-bearing flagstones as deposits formed within large Devonian lake systems – environments very different from some of the marine-style imagery used by Hugh Miller in his writings. However, Miller clearly recognised the specific quality of these sediments, suggesting correctly that they were something distinct. In addition, he recognised the varve-like quality of some of the sediments and he noted the exceptional quality of the preservation of the fossil fish.

Reconstructing a lost world

The Devonian lakes of the Orcadian Basin no longer exist, but the evidence they left behind is sufficient to reconstruct them in detail. Individually, the elements are simple: a sedimentary basin, lakes occupying low-lying areas, stratified water columns and organisms confined to habitable zones. Together, they form a coherent system.

Yet the Orcadian lakes represent only one part of the Devonian landscape. Beyond these basins, rivers and floodplains extended across what is now Britain. At sites such as Heol Senni quarry (see Plants, fish and floodplains: The Heol Senni story), plant remains preserved in fine sediments provide a complementary view of the same Devonian world.

From a split slab of stone, an entire landscape can be rebuilt – not imagined, but reconstructed from evidence.

The Orcadian lakes show how a vanished landscape can be reconstructed in remarkable detail from the rocks alone. Yet these were not the only lakes to shape Britain’s history. In Part 2, we move forward in time to the end of the last Ice Age, where the striking ‘parallel roads’ of Glen Roy reveal a very different kind of lake system – short-lived, dynamic, and dramatic enough to mislead even Darwin. The difference lies not in the processes themselves, but in their duration – a theme that becomes clearer when set alongside the transient lake systems of the last Ice Age.