Re-run due to high demand!
30 September 2017
Event phone: 0207 408 2000
Led by Simon Gould, Mearns Geoscience Ltd.
This fieldtrip will examine the excellent exposures along the River Esk, on the Aberdeenshire-Angus border in NE Scotland. North of Gannochy Bridge, the exposures provide a section through the northern limb of the Strathmore Syncline, which formed in the middle Devonian adjacent to the Highland Boundary Fault. The early Devonian, Lower Old Red Sandstone continental alluvial succession alternates between coarse-grained conglomerates, thick multi-storey sandstone packages and more thinly-bedded sheet sandstone units. The steeply dipping nature of the outcrop allow inspection of the temporal changes in the stratigraphic architecture. The early Devonian section overlies a substantial thickness of Siluro-Ordocivian marine sands, shales and basic igneous rocks of the Highland Border Complex, which record the deposition within, and the closure of the Iapetus Ocean.
The Glen Esk section is located approximately 1 mile north of Edzell, and is approximately 30 miles south from Aberdeen. The fieldtrip will consist of a gentle 3 mile riverside walk through the scenic and popular “Blue Door” walk of the Burn estate along a well-defined path that can be muddy in places. A small number of localities require decent of steep path to the river side, which may be slippery. This trip would be of interest to those working in continental alluvial reservoirs in the subsurface, but is mainly intended to show some of the local geology of the Aberdeenshire area.
“The Old Red Sandstone and Highland Border Complex at Glen Esk”, 29th April 2017.
The Highland Boundary Fault lies only a few miles south of Aberdeen. The landscape changes on crossing this boundary between the Scottish Highlands and the Midland Valley: sheep-populated high moorland of the grey Grampians gives way to the low-lying geometric farm fields of the Mearns, and ploughed earth changes from grey to red.
Similar distinctive changes can be seen along the length of the clearly-defined highland boundary from Stonehaven in the north-east to Helensburgh in the south-west. However, there are surprisingly few localities which offer a chance to walk the geological section from Lowlands to Highlands, and this section of the river North Esk is one of them. It is also a very pretty location, popular with salmon fishermen, dog-walkers and artists. On this day, however, the 24 attendees of this very pleasant geological field trip easily outnumbered other groups.
The riverside path lies within a private estate, but access is allowed through the “Blue Door” in the boundary wall. Simon provided a well-written field guide and knowledgeable commentary about the localities visited on the 3km walk from the Old Red Sandstone conglomerates at Gannochy Bridge across the Highland Boundary Fault to the ocean-floor greenstones of the Highland Border Complex at the Rocks of Solitude.
Old Red Sandstone conglomerates never fail to impress, and the effect of these near vertical pebble beds is enhanced by the fast-flowing river’s plunge through narrow gullies or elsewhere by deep pools of quiet dark brown water.
The Highland Boundary Fault zone itself is slightly less impressive and its location must be inferred rather than observed, as erosion of the softer lithologies of the basal Devonian (mudstone and tuff) and upper Highland Border Complex (silts and shales) leads to incomplete exposure.
The Highland Border Complex comprises remnants of the Iapetus ocean floor, closure of which led to the Grampian Orogeny. These remnants occur at various locations along the length of the Highland Boundary Fault, and alternative models exist to explain their age and emplacement mechanism. At Glen Esk they are represented by lithologies ranging from relatively undeformed thinly bedded ripple-marked sands, silts and shales to ocean-floor basalts which have undergone low-grade metamorphism. These latter are known as the Greenstone Series, their colour being due to chlorite content, and they display a high degree of deformation; they also contain large nodules of red jasper.
The “Rocks of Solitude” are part of the Greenstone Series, and mark not only the base of the Highland Border Complex but also the limit of the field excursion.
The field trip leaders had arranged an outstanding lunch at The Burn, the Georgian mansion house at the centre of the 200-acre estate we had been visiting, and which now serves as an academic study centre.
To sum up the day, this was a well-organised excursion to an important and pretty geological locality, culminating with an excellent lunch at an impressive country house. Highly recommended!
John Wham, 4th May 2017.
01356 648 281