The lost lakes of Britain (Part 2): The ‘parallel roads’ of Glen Roy – a vanished Ice Age lake system

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.

In Part 1, we reconstructed the long-lived Devonian lakes of the Orcadian Basin from their sedimentary record. Here, we turn to a much more recent and far more fleeting landscape. At the end of the last Ice Age, ice-dammed lakes formed across parts of Britain, leaving behind traces that are often subtle – but in Glen Roy, they are spectacular. The famous ‘parallel roads’ provide one of the clearest records of changing lake levels anywhere in the country, preserving the story of a landscape shaped over centuries, rather than millions of years.

Three perfectly level “roads” etched across a Highland valley puzzled early geologists and led Charles Darwin into one of his most famous mistakes. Today, they reveal a fleeting but dramatic story of ice-dammed lakes, shifting spillways and sudden drainage at the end of the last Ice Age (Fig. 1).

Fig. 1. The ‘parallel roads’ of Glen Roy, seen here on the slopes of Coire Bohaskey. These horizontal terraces mark former shorelines of an ice-dammed lake that occupied the valley during the last Ice Age. Their continuity across gullies and spurs reflects the level surface of the former lake. (*Image: David Dixon, via Wikimedia Commons. CC BY-SA 2.0. Cropped by the author*.)

A landscape that should not exist

Stand in Glen Roy and the first impression is one of quiet strangeness. The valley itself is unmistakably glacial – broad, steep-sided and U-shaped – but the eye is drawn again and again to three sharply defined lines that run along the slopes on either side. They are too straight, too level, too persistent to be ignored. They look, quite simply, like roads.

There is a moment, standing there, when the mind reaches for a human explanation. Perhaps these are old tracks cut into the hillside? Perhaps remnants of some long-forgotten route? But the scale defeats that idea almost immediately. The lines run uninterrupted for kilometres, maintaining a constant height as they cross spurs and gullies alike. No road could ignore the terrain so completely.

Walk closer and the illusion shifts again. These are not built structures, but subtle benches – gently sloping ledges cut into the hillside. In places they are broad enough to stand on comfortably; in others, they narrow to little more than a shelf. Yet always they remain level.

There are three principal levels, stepping up the valley sides with uncanny regularity. Together, they form one of the most striking and perplexing geomorphological features in Britain – a landscape that appears almost engineered, yet is entirely natural.

What are the ‘roads’?

Fig. 2. Close view of one of the ‘parallel roads’ in Glen Roy. Seen at ground level, the feature appears as a gently sloping bench cut into the hillside. Despite their road-like appearance, these terraces are natural shoreline features formed by wave action along the margins of an ice-dammed lake. (*Image: Robert Naylor, via Wikimedia Commons. CC BY-SA 2.0.)*

The key to understanding the ‘roads’ lies in recognising what they represent: former shorelines (Fig. 2).

Each terrace marks the surface of a lake that once filled the valley. At that water level, wave action – driven by wind across an open water surface – cut into the slope and reworked sediment, producing a gently sloping bench. Over time, these processes created a laterally continuous feature: a shoreline etched into the landscape.

This is not an unfamiliar process. Similar benches form today along the margins of modern lakes and reservoirs. What makes Glen Roy remarkable is the scale and clarity with which these features are preserved.

Even more striking is the fact that they occur at multiple, sharply defined levels. This immediately tells us that the water surface was stable for significant periods, and that the lake itself did not simply fill and drain once, but existed in a sequence of stages – each long enough to leave a visible mark.

In other words, Glen Roy preserves not one lake, but a record of several.

A lake without an outlet

Unlike the long-lived Devonian lakes discussed in Part 1, the Glen Roy system was geologically fleeting, yet it preserves its history with remarkable clarity (Fig. 3).

Fig. 3. Looking along Glen Roy, Lochaber. The broad, U-shaped valley reflects its glacial origin, but the former lake system responsible for the ‘parallel roads’ is not immediately apparent from ground level. Subtle horizontal benches can be traced along the valley sides, marking former lake surfaces during the last Ice Age. (*Image: Nigel Brown, via Wikimedia Commons. CC BY-SA 2.0.)*

At the end of the last Ice Age, around 12,000 years ago, this landscape looked very different. The great ice sheets that had covered much of Scotland were retreating, but glaciers still occupied many of the major valleys. One such glacier extended through Glen Spean, immediately to the west of Glen Roy.

This glacier acted as a natural dam. Water draining from the surrounding hills – from rainfall, meltwater and streams — could no longer escape freely westwards. Instead, it began to pond within Glen Roy, forming a large proglacial lake (Fig. 4). The valley, which today carries only a modest stream, was transformed into a substantial body of standing water.

Fig. 4. Thonak Tsho, a modern proglacial lake formed where meltwater ponds against glacial debris and ice within a mountain valley. Such lakes provide a useful modern analogue for the ice-dammed lakes that occupied Glen Roy at the end of the last Ice Age, when drainage routes were blocked and water accumulated within the valley system. (*Image: © Vyacheslav Argenberg, CC BY 4.0, via Wikimedia Commons.*)

It is worth pausing here to picture that change. The quiet valley floor would have been drowned beneath tens of metres of water, its sides lapped by waves driven by Highland winds. Tributary streams would have entered the lake as small deltas, building out into the still water. For a time, this would have been a very different landscape – one defined by water rather than rock.

But lakes do not simply continue to rise indefinitely. Once the water level reaches the lowest available escape route, it begins to overflow.

Reconstructing the vanished lake

The reconstruction in Fig. 5 helps to visualise a landscape that no longer exists. With the valley blocked by ice, a large lake filled Glen Roy, its surface perfectly level and extending for many kilometres.

Fig. 5. Reconstruction of Glen Roy during the late stages of the last Ice Age. A glacier occupying Glen Spean blocked drainage to the west, forming an ice dam and allowing a lake to fill the valley. The three ‘parallel roads’ mark former stillstands of the lake surface.

Along its margins, wave action cut into the slopes, forming the terraces we see today. Each represents a period during which the lake level remained stable long enough for a shoreline to develop. Seen in this way, the ‘roads’ are not paths through the landscape, but the edges of a vanished lake – a set of horizontal lines marking former water surfaces, now preserved as geomorphological features.

The question, then, is not simply how the lake formed, but why it existed at several distinct levels.

Spillways and shifting lake levels

The level of a lake is determined not by how much water flows into it, but by the height of its lowest outlet. In Glen Roy, that outlet was not a single river channel, but a series of cols – low points in the surrounding landscape through which water could spill.

These cols are easy to overlook in the modern landscape. They are often broad, gentle passes rather than sharply defined channels. Yet during the late glacial period, they played a decisive role. Once the lake surface reached the height of a particular col, water began to flow out across it, preventing the lake from rising further.

The lake level would then stabilise. Waves would work along the shoreline, cutting a terrace into the slope. Over time, a distinct ‘road’ would be formed.

Crucially, the position of the ice dam was not static. As the glacier advanced, retreated or thinned, it could block some outlets while opening others. A previously active spillway might be sealed off by ice, forcing the lake to rise until it found a new escape route at a higher level.

How the system evolved

What took millions of years to build in the Orcadian Basin was compressed here into a sequence of rapid changes, driven by the shifting position of ice, rather than long-term sediment accumulation. This sequence shows how a single valley could record multiple lake levels (Fig. 6).

Fig. 6. Evolution of the Glen Roy lake system. (A) prior to full blockage, Glen Roy drains normally. (B) ice in Glen Spean forms a dam, creating a lake controlled by a low-level overflow. (C) as the ice blocks this outlet, the lake rises to a higher spillway. (D) further ice advance forces the lake to its highest level. Each stillstand produced a distinct shoreline.

Each time the ice blocked a lower outlet, the lake rose to the next available spillway. Each stillstand produced a new shoreline, preserved as one of the terraces visible today.

What we see now is the cumulative result of these changes: a stepped pattern of horizontal lines, each marking a former lake surface. The ‘parallel roads’ are, in effect, a history of changing drainage conditions written onto the sides of the valley.

A connected system: Glen Roy and Glen Gloy

Glen Roy does not stand alone. Nearby Glen Gloy preserves evidence of a separate, earlier lake at a lower level, drained through its own spillway (Fig. 7).

Fig. 7. Looking east-northeast up Glen Gloy, a neighbouring valley to Glen Roy that hosted a separate ice-dammed lake at a lower level. A single, subtle terrace can be traced along the left-hand slope, marking a former shoreline and providing important evidence for the evolving Lochaber lake system. (*Image: George Brown, via Wikimedia Commons. CC BY-SA 2.0.)*

Seen together, these valleys reveal a dynamic and interconnected system. As ice margins shifted, lakes formed, expanded, merged and drained, their levels governed by a changing network of overflow routes. What might at first appear to be a simple set of terraces is, in fact, the product of a complex sequence of events (Fig. 8).

Fig. 8. Plan-view reconstruction of the Lochaber lake system. Ice occupying Glen Spean impounded water in Glen Roy, while neighbouring Glen Gloy hosted a separate lake at a lower level. Changing overflow routes controlled lake levels.

Reading the landscape

Fig. 9. A spur in Glen Roy crossed by one of the ‘parallel roads’. The terrace maintains a constant level as it cuts across the projecting slope, showing that it records a former lake surface rather than a constructed feature or irregular erosion. (*Image: Richard Webb, via Wikimedia Commons. CC BY-SA 2.0.)*

Once the lake interpretation is understood, the landscape becomes readable. The terraces maintain a constant height across the valley, ignoring the underlying topography. They cut across spurs and minor valleys without deviation, tracing a perfectly level line through an otherwise irregular landscape. This is exactly what one would expect of a water surface. It is also one of the strongest pieces of evidence against earlier interpretations that invoked marine processes or human construction.

With a little practice, it becomes possible to ‘see’ the former lake – to trace its surface across the valley and imagine the water that once occupied it. The terraces are no longer puzzling lines, but the edges of something that has vanished.

Darwin’s “gigantic blunder”

When Charles Darwin visited Glen Roy in 1838, the concept of widespread glaciation was not yet widely accepted. Faced with these striking horizontal terraces, he interpreted them as raised marine shorelines – beaches formed when sea level stood higher than today.

It was, in many ways, a reasonable conclusion. The features resembled coastal terraces, and the idea of changing sea levels was already part of geological thinking. Without a clear understanding of glacial processes, the idea of a former sea filling the valley seemed plausible.

Yet the interpretation contained difficulties. The terraces occurred at multiple levels, far inland, with no clear connection to a broader marine landscape. Something did not quite fit. The missing piece was the role of ice. Once it was recognised that glaciers could act as dams, impounding lakes at high elevations, the Glen Roy terraces could be reinterpreted in a new light. The features that had seemed so puzzling suddenly made sense.

Darwin later acknowledged his error, referring to his interpretation as a “gigantic blunder”. Yet the episode remains a powerful reminder that even the most careful observations can be misinterpreted when key processes are not yet understood.

A fleeting landscape

The lakes of Glen Roy existed for only a brief interval at the end of the last Ice Age, perhaps lasting only a few centuries. As the climate warmed and the glaciers retreated, the ice dam in Glen Spean weakened and eventually failed. The lakes drained, leaving the valley once again open to normal river flow.

What remains is a set of fossilised waterlines – traces of surfaces that once existed but are now long gone.

Seeing what is no longer there

Fig. 10. The ‘parallel roads’ of Glen Roy seen in their wider landscape setting near Coire Bohaskey. Once recognised, the terraces can be traced across the valley sides, marking the former shorelines of a vanished ice-dammed lake. (*Image: David Dixon, via Wikimedia Commons. CC BY-SA 2.0.)*

The parallel roads of Glen Roy are more than a geological curiosity. They are a reminder that landscapes are not fixed, but transient – shaped by processes that may operate intensely for a short time and then vanish.

To walk along one of these terraces is, in a sense, to walk along the shoreline of a vanished lake. Glen Roy captures a moment in geological time – a landscape that existed only briefly, yet left an imprint that is still unmistakable today. But it is only one part of a much wider pattern. In Part 3, we step back to look at Britain as a whole, tracing the broader network of Ice Age lakes and the lasting impact they had on river systems, drainage patterns and the landscapes we see today.

Other parts in this series
The lost lakes of Britain (Part 1): The Orcadian lakes – reconstructing Scotland’s lost Devonian world
The lost lakes of Britain (Part 2): The Parallel Roads of Glen Roy – a vanished Ice Age lake system
The lost lakes of Britain (Part 3): Landscapes that vanished, rivers that remain