Story of the landscape and the underlying geology of Strathearn – the broad valley of the River Earn – in Perthshire, Scotland. Specifically, the area around the small town of Comrie, located between Crief and Loch Earn.
The author explored the area as a participant on a geological mapping course, run by Dr Nick Chidlaw in the Spring of 2026.
Fig 1 Overall location in Scotland
Contrasting Landscapes
Landforms to the north of Comrie – opening view is south
Landforms to the south of Comrie – opening view is north
Highland Boundary Fault
The area is crossed by the Highland Boundary Fault (HBF) passing roughly east to west, just to the south of Comrie town. The fault is not a simple line, as it may appear on maps, but a zone, over 1km wide at Comrie, of highly crushed rock, with multiple parallel faults along which vertical and horizontal displacement has occurred.
The fault zone in the Comrie area has a steep dip to the NW, with a vertical displacement of the order of 2500 to 3000m It is a reverse fault, whereby the hanging wall has moved up relative to the foot wall.
In this Google Earth image, the zone of the Highland Boundary Fault is depicted by the fuzzy green line. Running from the North Sea coast at Stonehaven, the fault continues beyond Loch Lomond, passing beneath the Firth of Clyde, Bute and Arran – see Fig 1.
Exposures in/near the Fault Zone
Look along Glen Artney, eroded along the line of the Highland Boundary Fault Zone. as it continues SW away from Comrie.
West Cowden. Highly sheared rock (microdiorite/diorite), with a distinct surface along which movement has occurred, dipping around 27° to the WNW.
The rock on the northern side of the fault zone is older (Edicaren to Mid-Ordovican) that the rocks on the south – Devovian.
At the same location, a private garden at West Cowden, ochre stained sheared microdiorite/diorite.
Ochre staining like this is a common feature of rock in.a shear zone.
Highly fractured Devonian sandstone in a small track side exposure, near the Mill of Fortune, on the southern side of the fault zone.
More frsctured rock. The fabric of this showing a distinct NW dip , in keeping with the direction of dip of the Highland Boundary Fault Zone.
This location is just a few tens of metres futher along the track from the previous exposure. The rock here appears to be andesite – an extrusive igneous rock – more on that later.
When was the fault active?
The last significant movement on the fault has to be post the age of the Devonian rocks that are clearly displaced by it. The main displacement is considered to be mid-Devonian during the Acadian Orogeny, which is phase of the Caledonian Orogeny, related to the closure of the Iapetus Ocean.
The fault corresponds with the terrane boundary between the Grampian Terrane and the Midland Valley Terrane. Along this boundary significant horizontal displacement occurred long before vertical movement associated with the Acadian Orogeny.
Earthquake House Comrie, constructed in 1874 to house a Pinned Seismometer (see below), developed by the Irish civil engineer Robert Mallet (considered the father of seismology). Comrie has a long history of very minor earth tremors, giving in the names ‘Shaky Town’ and ‘Earthquake Capital of the UK’. A 4.8 magnitude event in 1839, which caused structural damage and opening fissures up to 200 yards long at Amulree, was the stimulus for local people to establish earthquake monitoring here. The most recent (to June 2026) tremor was Oct 20th 2025, a 3.3 magnitude event, centred at Pubil 36km NW of Comrie, at a depth of 3km. An epicentre that corresponds to the descending HBF Zone, with its NW orientated plunge. These minor tremors relate to ongoing crustal recovery after the unloading of the thick glacial ice that covered the area in the last glaciation, ending around 12K years ago.
Earthquake House fell out of use from 1911, until the BGS installed modern equipment in the 1980’s following the 1977 designation of building of special historic interest – the smallest building in Europe to receive the designation. It continues to record seismic activity, with information on events made available to the public by its custodian Chris Palmer. It is not linked to the BGS Seismic Array for the UK. An automated station in Glen Lednock performs that role.
Pinned Seismometer on the floor of Earthquake House. Two planks orientatated N-S and W-E carried carried cylinders of different sizes. These fell into the sand when a tremor occurred. Based on which cylinders fell, the magnitude and to some location of the earthquake could be determied.
Geological history of Comrie area
The rocks exposed either side of the Highland Boundary Fault Zone, are introduced here alongside the probable geological history of the area. A time spanning from the Ediacaren (late Precambrian) to the Devonian.
A possible reconstruction of the supercontinent Rodinia [1], for around 750Ma in the Tonian period (1000 to 720Ma) of the late Precambrian. The crust of the future Scotland and the northern half of the island of Ireland lay within this continental mass at a position about 30° south of the equator.
By the late Tonian, Rodinia was beginning to break apart and the crust of the northern part of the future British Isles lay within a zone of crustal extension. This led to the formation of sedimentary basins and periodic marine incursions.
These sedimentary conditions were broadly maintained through the late Tonian, Edicaren, Cambrian and early Ordovician, resulting in some 25km thickness of sediments forming. The resulting rocks are known as the Dalradian Supergroup. Essentially the rocks of the Grampian Terrane.
In the Comrie area, the rocks to the north of the Highland Boundary Fault Zone, belong the Southern Highlands Group of the Dalradian Supergroup.
During the Ediacaren the zone of crustal extension finally broke and oceanic plate of the Iapetus Ocean formed between Laurentia and Gondwana. The second reconstruction[2] depicts this situation around 550Ma in the late Ediacaren. The future Scotland is now located on a passive continental margin, which continues north and south for thousands of kilometres.
This continental edge location led to the deposition of mudstones formed from sediment eroded from Laurentian and was washed out over the shelf into the deeper oceanic basin. These mudstones underwent metamorphism in the later Grampian Orogeny to produce a slate – the Aberfoyle Slate.
Sedimentation later came to be dominated by deposition from turbidity flows, in which sediment is transported down the continental slope as a chaotic slurry, fanning out over the ocean floor, often covering distances of hundreds of kilometres. The resulting rocks are known by their German name as Greywackes. Coarse sandstones, in which the poorly sorted grains sit within a ‘mud’ matrix. The Ben Ledi Grit, which follows the Aberfoyle Slate, was produced in this way.
Some preserved sedimentary structures indicate sediment flowed from the NW.
Google Earth image of present-day continental shelf of western Europe. Canyons descend to the deep ocean floor, over which fans of sediment splay out.
Deposition of the Aberfoyle Slate and the Ben Ledi Grit, spans a period of time from the Early Ediacaran (c.595Ma) to the Middle Ordovician (c.470Ma).
There is no distinct divison between them, for example an unconformity, they effectively transition one to the other, with greywackes occuring more towards the top of the Aberfoyle Slate and slates being more common towards the base of the Ben Ledi Grit,