Balmaha, Loch Lomond: Walk the Highland Boundary Fault

The rocks at Balmaha contain evidence of collision on a continental scale, helping to explain the strikingly different landscapes seen in Lowland and Highland Scotland

1. Loch Lomond and Inchcailloch island from above The Old Manse, Conic Hill. Taken facing west, standing on Highland Border Complex serpentine breccia.
Image: Simon Cuthbert.

Balmaha is an excellent example of how separate pieces of evidence such as the shape of the landscape, rock exposures and tiny details in the rocks can be brought together to give a picture of forces and movement on a continental scale. The fact that all of this can be done in the picturesque setting of Loch Lomond makes the site even more special.

Balmaha lies on border between the Scottish Lowlands and Highlands, which marks a change in the landscape across a ruler-straight line that runs across the country, from Stonehaven to Bute. This line, the Highland Boundary Fault, is a great fracture in the Earth’s crust. Mountains that are over 800 metres high rise dramatically to its north, while the ground to the south has subsided, forming the “Midland Valley Basin” in which sedimentary rocks piled up during Devonian, Carboniferous and Permian times (420-250 million years ago).

On the northern side of the Highland Boundary Fault the rocks are metamorphic; Dalradian slate and schist, named after the ancient Gaelic kingdom of Dál Riata. These rocks were originally sediments laid down on a continental shelf about 600-500 million years ago. Later, when continental collisions formed the Caledonian mountain chain, the sedimentary layers were intensely folded and heated, converting them to tougher metamorphic rocks. You can see the Dalradian metamorphic rocks at other Best Places, including Schiehallion and Portsoy. These rocks have resisted erosion more than the softer rocks in the Midland Valley to their south, explaining why there is such a dramatic change from high mountain scenery to lowlands across the Fault.

Sandwiched between the Dalradian and Midland Valley rocks lies a third, exotic group of rocks known as the “Highland Border Complex”. It forms narrow slivers along the Highland Boundary Fault. Some of these rocks resemble those found under the ocean floor: they include serpentine and marble of Ordovician in age, perhaps as old as 470 million years. The serpentine is a “breccia” – a mass of angular fragments. Its origin is obscure, but it may have been formed by submarine landslides on a steep slope on the ocean floor. These rocks are very different from the rocks on either side of them. This poses a question: How were such different groups of rocks, from such different places on the Earth, brought together?

At Balmaha we can piece together some of this ancient jigsaw puzzle. Just north of the village there is a thick layer of conglomerate, sometimes known as “pudding stone”, about 420 million years old. These striking rocks are part of the Old Red Sandstone sedimentary rock sequence and are made from round white cobbles set in a matrix of reddish sandstone, like big plums in a pudding! They lie upon older rocks from the Highland Border Complex and the contact between them is an ‘unconformity’, or ancient erosion surface. The Highland Border Complex has been thrust up from the ocean floor to form a mountain range on the continental margin, and was then eroded, then later buried under river gravels that form the conglomerate. The whole sequence was then folded, tipping the layers into their present near-vertical attitude. The northern side of the serpentine lies against a fault (the Gualan Fault – a branch of the Highland Boundary Fault) along the northern flank of Conic Hill.

On the north side of this fault there are younger Old Red Sandstone rocks, of late Devonian age. These are brighter red with angular fragments of gritty schist, transported by powerful streams from the mountainous terrain to the north. These layers lie unconformably on top of the folded and metamorphosed Highland Border Complex rocks. Further north at Arrochymore Point you can find waxy green serpentine and red jasper, likely from oceanic crust. Here we reach the limit of the Highland Border Complex, before meeting the Dalradian schists in the craggy flanks of Ben Lomond.

The contrast between the two sections of Old Red Sandstone rocks here has triggered a geological controversy. There are no fragments of Dalradian rocks in the Lower Old Red conglomerates and the Highland Border Complex serpentine breccias. Some geologists have taken this to mean that a great distance separated the Dalradian from the Highland Border Complex in the Ordovician and even the early Devonian, and that they were brought together later by plate tectonic movements. Perhaps these two “terranes” were brought together along the Highland Boundary Fault with one sliding sideways into position like a great stone liner coming into port. Other geologists would say that these groups of rocks, despite their differing characteristics, have never been widely separated. They suggest the lack of Dalradian fragments in the older conglomerates and breccias is because no erosion was happening, or because the sediment was not sourced from the Dalradian rocks but coming from somewhere else. Even after over a century of research, this remains controversial! Maybe you’ll be the one to find a workable answer?

Text contributed by Simon Cuthbert & Gavin Eardley

Find out more
There is an excellent scenic trail along the shore of Loch Lomond from Balmaha to Millarochy Bay that gradually reveals the clues to the nature of the Highland border geology. A pamphlet describing the geology along the trail, published by the Geological Society of Glasgow, is available from the visitor centres at Balmaha and Millarochy Bay, or download from https://geologyglasgow.org.uk/geoconservation/rigs/balmaha/

Another good way to get the big picture of the geology at Balmaha is to take the footpath Balmaha to Conic Hill. If you look westwards when you reach the ridge crest you will see that the ridge continues across Loch Lomond as a line of islands. The ridge and the narrow terrace to its north are Lower Old Red conglomerate and sandstone and the northern flank of the ridge and the low ground along its base are Highland Border Complex serpentine and the Gualan Fault.

National Park website: http://www.lochlomond-trossachs.org/

B. J. Bluck (2010) The Highland Boundary Fault and the Highland Border Complex. Scottish Journal of Geology, 46, 113-124.
Stephenson, D, and Gould, D. 1995. British regional geology: The Grampian Highlands. Fourth edition. Keyworth, Nottingham: British Geological Survey
P.W.G. Tanner (2008) Tectonic significance of the Highland Boundary Fault, Scotland. Journal of the Geological Society, London, Vol. 165, pp. 915–921.
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