By John Lewington FRSGS, RSGS Volunteer (Fair Maid's House and Collections) 

In August 2024, the Parallel Roads of Glen Roy were one of three new sites added to a list of internationally important geological locations by the International Union of Geological Sciences.

The other two were the Rum Igneous Complex, and the Barrow Zones in Glen Esk in the Eastern Highlands, joining a list that already included well known sites such as Siccar Point in the Borders and the Moine Thrust in the north-west Highlands.

Coincidentally in 2024, RSGS was donated an original copy of an 1818 article by Sir Thomas Dick-Lauder, and within it was superb illustrations and a map of the Parallel Roads of Glen Roy. First published in the Transactions of the Royal Society of Edinburgh, it was one of the first scientific papers on this remarkable glaciological phenomenon.

This is not the first blog RSGS has featured about Glen Roy – in October 2021, RSGS Writer-in-Residence Jo Woolf, wrote about “Charles Darwin and the ‘horrid shelves of Glen Roy”, but that was before Darwin had learned better!

Glen Roy is especially notable because its distinct moraines and polished rock surfaces led to the first serious consideration of the idea that there were once glaciers in Scotland, which of course we know today to be true…

Plate IX - A map showing the different lines of horizontal shelves in the Glens of Lochaber

Formation of the Parallel Roads (Glaciers and Lakes)

An icefield developed to the west of Glen Roy and the Great Glen during the Loch Lomond Stadial, with a further ice centre to the south, over Rannoch Moor. Glaciers flowed eastwards along the glens from the icefield in the Western Highlands. One tongue of ice blocked the entrance to Glen Roy, while another extended eastwards to block lower Glen Gloy, and another also extended eastwards to block lower Glen Roy. Here it met a glacier that had extended into the middle section of Glen Spean from the Loch Treig valley. The blocking of Glen Spean led to a lake being impounded with a surface level of about 260 metres above sea level. This lake overflowed eastwards through the Loch Laggan valley to drain eventually into the River Spey. As the glacier advanced up lower Glen Roy, it cut off a lake in this valley and the rising water eventually found its way across a pass at 325 metres above sea level.

Plate I - View down Glen Gloy from the hills at its upper extremity from point a in the map.

A further advance of the glacier up Glen Roy blocked this exit for the water, causing the lake to rise to 350 metres and drain across the low ground at the head of the Roy and Spey glens. A separate lake, impounded in Glen Gloy at a level of 355 metres above sea level, drained through the head of this glen, crossing a pass into Glen Roy. As the ice retreated, these overflows were unblocked in reverse order. Finally, the ice dam near Spean Bridge broke, and the 260-metre lake drained away under the glacier very suddenly and rapidly towards the Great Glen.

Plate II - View of the mouth of Glen Roy and Glen Spean taken from above Inch from point b in the map.

The Parallel Roads (Lake Shorelines) 

The Parallel Roads represent the shorelines of the ice-dammed lakes. Typically, they are narrow benches (several metres wide) cut into the bedrock of the hillsides and in places covered by remnants of lake beach gravel. They extend along much of Glen Roy and Glen Gloy and parts of Glen Spean.

It is likely they formed through a combination of intense frost weathering and wave action along the lake shore zone. The control of lake levels by the different cols allowed the lakes to persist for sufficiently long periods such that even the hard, Precambrian bedrock was broken up and eroded. 

Plate III Figure 1 - The gap as seen from Glen Collarig taken from point c in the map.

Plate III Figure 2 - The gap as seen from Glen Roy taken from point e in the map.

Later Lake Drainage

As the climate improved about 11,500 years ago, the glacier damming the lakes retreated and the lakes drained through a sequence of glaciers bursts, or very rapid drainage or by the Icelandic term ‘jokulhlaup’. Effectively, the buoyancy of the water floated the glacier tongue off its bed. This happened suddenly, allowing the lake waters to drain in great floods under the ice. The biggest glacier burst took place after the ice dam had retreated to Spean Bridge, and the lake was at the level of the 260 metres Parallel Road; at this stage some 5 cubic kilometres of water are believed to have escaped under the ice to Loch Ness and into the sea at Inverness. As the ice surface declined, lower lakes were dammed; these also seem to have drained by glacier bursts directed either northwards to Loch Ness or south-westwards to Fort William.

Plate IV- View up Glen Roy taken from the gap from point d in the map.

The glacier burst that drained the 260 metres lake seems to have taken place under the ice along the Spean Gorge. Some of the later, smaller glacier bursts were directed under the ice into the gorge of the River Lundy. Most of the later lakes were probably short-lived, but one at 113 metres above sea level formed a locally-preserved shoreline and was probably longer lived.

Plate V - View of the head of Glen Roy taken from point f in the map.

Interpreting the Landscape – Changing Times and Ideas 

The Parallel Roads provided easy routes for travellers in a region where communication was difficult, long before road building began early in the 18th century. They were attributed variously to the activities of the mythical Gaelic hero-giant Fingal (of Fingal’s Cave fame) and to the work of the early Kings of Scotland. Certainly, by the late 18th century and early 19th centuries the Parallel Roads were one of the popular sights of Scotland and were being visited by the local gentry, such as the Grants of Rothiemurchus, as recorded in ‘Memoirs of a Highland Lady’.

The controversy over the formation of the Parallel Roads illustrates the development of science in light of two 19th century discoveries – that sea-level can rise and fall in response to Earth movements, and that glaciers had existed in Scotland in the geologically recent past. The view that the Parallel Roads were old marine shorelines was championed by the young Charles Darwin, who, fresh from his voyage to South America, had been deeply impressed by the uplift of the Chilean seashore by recent earthquakes. The originally less popular view, that the Roads were lake shorelines, came to prominence very shortly afterwards following the visit to Scotland of the Swiss geologist Louis Agassiz in 1840. Agassiz suggested a key mechanism for the otherwise puzzling formation of lakes in these glens was damming by glacier ice; this was put on a firm footing by Thomas Jamieson some twenty years later. The interest in the Parallel Roads extended to government circles, and the Ordnance Survey diverted some of its effort into specially surveying the Parallel Roads, showing that they were indeed (almost) horizontal and could be lake shorelines.

Historically, also, the landforms in Glen Roy and Glen Spean, near Loch Treig, played a key part in convincing Agassiz of the reality of the former existence of glaciers in Scotland. They provided the crucial field evidence that he needed to confirm the theory of a great ice age following his visit, so he sent a now famous letter from Fort Augustus, announcing publically the glacial theory. This world exclusive appeared in “The Scotsman” on 7th October 1840.

Plate VI - View of the entrance to Loch Treig taken from the top of Tom-na Fersit in Glen Spean from point g in the map. 

Plate VII - Fig 1. the barrier of rock of the Valley of Subiaco...Fig 2. Section of the barrier rock...Fig 3. Section of the Valley of Subiaco...Fig 4. Map of the Valley of Subiaco...Fig 5. Theory of formation of the shelf...Fig 6. Glen Roy pseudo shelves. 

An Area of International Importance for Earth Heritage

The landforms and deposits in Glen Roy, Glen Gloy and Glen Spean are an internationally important part of Scotland’s Earth Heritage. They provide the clearest evidence in Britain for the formation and catastrophic drainage of a series of ice dammed lakes at the end of the last glaciation. The features were first recognised over 150 years ago, and have subsequently appeared as classic examples in many textbooks.

The Parallel Roads are exceptional in terms of the extent, clarity and degree of development of the glacial lake shorelines, as well as for the range of associated landforms and deposits preserved in a relatively compact area. These record in detail the processes of landscape development both during and following successive stages of glacial lake development and catastrophic drainage.

The Parallel Roads and associated landforms are protected as a Site of Special Scientific Interest (SSSI). Part of Glen Roy is also a National Nature Reserve. The principal pressures on the Earth heritage of the area are from blanket afforestation, which hides the landforms, and quarrying for sand and gravel, which destroys them. Other activities such as building or construction of roads and tracks can have a locally damaging impact which can be cumulative over time.

For further reading see D Peacock, J Gordon and F May. Glen Roy  -a landscape fashioned by geology Scottish Natural Heritage, 2004