The Highland Boundary Fault is a geologic fault that traverses Scotland from Arran and Helensburgh on the west coast to Stonehaven in the east. It separates two distinctly different physiographic regions: the Highlands from the Lowlands, but in most places it is only recognisable as a change in topography.[1]
Aligned southwest to northeast, from Lochranza on Arran it bisects the Isle of Bute, and crosses the south eastern parts of the Cowal and Rosneath Peninsulas as it passes up the Firth of Clyde. It comes ashore near Helensburgh then continues through Loch Lomond. The loch islands of Inchmurrin, Creinch, Torrinch, and Inchcailloch all form part of the Highland Boundary Fault.[2]
From Loch Lomond it continues to Aberfoyle, then Callander, Comrie and Crieff. It then forms the northern boundary of Strathmore and reaches the North Sea immediately north of Stonehaven near the ruined Chapel of St. Mary and St. Nathalan.[3] To the north and west lie hard Precambrian and Cambrian metamorphic rocks: marine deposits metamorphosed to schists, phyllites and slates. To the south and east are Old Red Sandstone conglomerates and sandstones: softer, sedimentary rocks of the Devonian and Carboniferous periods.[3]
The Highland Boundary Fault was active during the Caledonian Orogeny,[1] a plate tectonic collision which took place from Mid Ordovician to Mid Devonian periods (520 to 400 million years ago), during the closure of the Iapetus Ocean. The fault allowed the Midland Valley to descend as a major rift by up to 4000 metres and there was subsequently vertical movement. This earlier vertical movement was later replaced by a horizontal shear. A complementary fault, the Southern Uplands Fault, forms the southern boundary for the Central Lowlands.[3]
The Highland Boundary Fault is a geological fault that traverses Scotland from Arran and Helensburgh on the west coast to Stonehaven in the east. It separates two distinctly different regions: the Highlands from the Lowlands.[1][2]
The geological history of Scotland can be understood as the result of two great tectonic events. The first was the formation of the global super-continent Pangaea, the second was the break-up of Pangaea to form the continents we know today.
The fault is the middle of three great faults which run south-east to north-west across Scotland. In the north there is the Great Glen Fault, and in the south the Southern Uplands Fault. Further south, just over the border with England, is the Iapetus Suture, where the palaeo-Iapetus Ocean closed. These four major geological signs mark the ancient history of Scotland.
The Highland Boundary Fault was active during the Caledonian Orogeny.[1] This was a plate tectonic collision which took place from Mid Ordovician to Mid Devonian periods (520 to 400 million years ago), during the closure of the Iapetus Ocean.
The fault allowed the Midland Valley to descend as a major rift by up to 4000 metres and there was subsequently vertical movement. This movement was later replaced by a horizontal shear. A complementary fault, the Southern Uplands Fault, forms the southern boundary for the Central Lowlands.[3]
   | Loch Lomond - Highland Boundary Fault - Scottish Geology Website |
| | Highland Boundary Fault - Overview of Highland Boundary Fault |
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