When the stress and strain of a Monday morning gets you down, sit back and relax with some geology!

A brief geology of the Easter Ross district.

The geology of Easter Ross may not have the diversity of its western neighbour Wester Ross, but nonetheless has some exciting geology, and some important outcrops.  The area is dominated by the Devonian strata that outcrops across most of the Black Isle, and extends as far west as Contin, on the road to the west coast.  This Devonian strata is historically termed the Old Red Sandstone, and unsurprisingly comprises of typically red sandstone units.  However first, we will explore the older rocks; those known as the Caledonides, or more commonly the Moine Succession, for these rocks form the basement for the overlying younger sedimentary rocks. 

The Moine Succession can be divided into three basic divisions, the Morar Division, the Glenfinnan Division, and the Loch Eil Division, all taking their names from well-known locations to the west.  This sub-division is not on a purely stratigraphic basis, with field relationships and features having been studied in great detail.  The Moine is thought to have originated as sedimentary rocks laid down onto the Lewisian basement between 1,500 and 1,000 ma.  A series of orogenies has resulted in the metasedimentary succession we see today, the last of which was the Caledonian Orogeny.  The succession broadly youngs from the west to the east, so as the Loch Eil is the youngest division, this is the unit that outcrops in the wets of Easter Ross.  Within the Loch Eil Division, the dominant lithology is psammite, such as a quartzofeldspathic granulite, with some bands of pelitic and semi-pelitic shists.  Metamorphism was generally of a medium grade, although is thought to be of a higher degree than that of the neighbouring Morar Division, as the rocks appear more flaggy.  Hints of the original sedimentary rock prior to metamorphism such as cross bedding can be found, but are fairly scarce.  The Moine appears a grey, unimpressive succession, and compared to the exposures offered further west does not offer huge potential for the amateur geologist.  However, the history of tectonic events recorded by mineral assemblages in these metasedimentary rocks, particularly in the pelitic units, is of great use to metamorphic geologists seeking to unravel the history of past orogenies and metamorphic histories. 

From this deep-seated, old basement we travel east, leaving the high peaks of the Munro’s behind us onto the relatively flatter coastal regions of Easter Ross and the Old Red Sandstone outcrops.  The change in topography denotes a change in bedrock; from hard metamorphic rocks, to more recently deposited sedimentary rocks, the most prominent of which is the Old Red Sandstone.

Those interested in sedimentary geology and palaeontology will undoubtedly find the Old Red Sandstone (ORS) has more to offer than the Moine Succession described above.  The ORS extends from Contin in the west to the Moray Firth, and forms the peninsulas of the Black Isle and Tarbet Ness - Tain.  The ORS is divided into the Lower, Middle and Upper divisions, of which all three outcrop in Easter Ross.  The general depositional environment of the ORS as a whole is far different from the environment we live in today.  The climate was tropical to sub-tropical as what is now Britain lay between 30 and 10 deg S.   Deposition occurred in discrete basins, the extent of which may have been fault controlled.  The sediment was provided from large rivers flowing into freshwater basins from the west, south and east.  Evidence for this is provided in the numerous sedimentary structures that can be seen within the deposits, and the faunal assemblages provided.  The ORS in Easter Ross forms a south west plunging syncline, with its axis across the spine of the Black Isle, and north to towards Inver, east of Tain. 

The Lower ORS, being the oldest of the ORS units, outcrops in the west of the area adjacent to the Moine.  This boundary is in part an unconformity, and in part a faulted contact.  The Lower ORS comprises of a basal conglomerate (Kilmorack Group) which is visible at the bottom of Stathconon, and Strathglass and passes into the argillaceous sedimentary rocks visible at Strathpeffer, termed the Strathpeffer Group.  These comprise of grey and olive mudstones, flags and shales, some of which are bituminous and fetid, with thin limestones and calcareous mudstones.  The fetid beds give off a smell of hydrogen sulphide or paraffin when struck with a hammer, and give rise to the spa waters of Strathpeffer, the local Victorian health giving watering hole.  Fossils are not common with the Lower ORS, although some algal remnants have been identified.    

The Middle ORS has a far greater potential for fossil preservation, and locations such as Edderton, Eathie and Cromarty should be scribbled in the notebooks of any palaeontologists venturing north.  Similarly to the Lower ORS the Middle ORS comprises of a basal conglomerate unit that forms local freestanding hills such as Cnoc Fyrish and Knock Farril.  The conglomerates contain clasts of Torridonian and Cambrian rocks as well as Dalradian rocks, suggesting an element of eastern and western fluvial flow during deposition into the central alluvial plain.  The upper part of the Middle ORS comprises of the Millbuie Sandstone and Strath Rory Groups both of which contain fish beds, either as nodular beds, or otherwise.  The lithologies are predominantly fluvial sandstones and conglomerates, with calcareous shales.  The faunal assemblages bear similar characteristics to the well-documented Achanarras fauna of Caithness.  These primitive fish are preserved as whole specimens, and as single head plates and scales, and can be found within calcareous nodules, and shaley beds.

The Upper ORS outcrops in Easter are located around the axial region of the syncline and comprise a large section of the Tarbet Ness – Tain peninsula.  These are predominantly red to yellow sandstone beds, with cross-bedding and occasional pebbly horizons.  No fish beds are recorded, although scales and other fish remnants have been recorded. 

 

Coming soon......Slivers of the Mesozoic, Jurassic windows on the Easter Ross foreshores.


Hugh Miller – The father of the Old Red Sandstone - and he was from Ross-shire!