The most ancient rocks in the Brittish Isles are known as the Lewisian and outcrop on the Outer Hebrides and along the west coast of the Northern Highlands.
The Moine Supergroup in the Highlands lie between the Great Glen Fault (Inverness to Fort William) and the Moine Thrust (Durness - Skye). These Neoproterozoic sequences were deposited between c. 1000Ma and c. 870 Ma. These have been changed due to extreame pressures and temperatures and have been squeezed into great folds. The rocks of the Strathy Complex croups out within one of these folds, the Swordly Nappe. The outcrop of the complex roughly correlated to a large magnetic anomaly that extents 9km offshore. The meta-igneous (ie originated from igneous rocks) of the complex are both lithologically and geochemically different from both the other Moinian and basement rocks. The complex is bounded by brittle faults and by a ductile thrust (low angle fault) in the west.
The rocks are dominated by K-feldspar free, silicious grey gneisses with subordinate hornblende gneiss, rare ultramafic units, garnet/staurolite/sillimanite gneiss and marble. Later cross cutting is dominated by trondhjemitic pegmatites and amphiboloites. The rocks have suffered three phases of deformation and amphibolite facies metamorphism. To the non-geologist this simply means that the rocks are hard and fairly complicated!
Latest research points to the rocks of the Strathy Complex having been deposited in a region where there was active volcanoes (arc-related) next to a warm sea (marble). The rocks are probably sub-Moine and thus can be considered to be Archean, yet they are distinct from all the other outcrops of Archean rocks in Scotland. Some authors have suggested that the Strathy Complex was related to the Grenvillian mountain building event (1100-1000Ma) that took place on the Rondianian Supercontinent.
Ancient Lochs: The East - West Contrast
The rocks to the east of the river represent as significant contrast to those in the west. East of the river the rocks form part of the same sequences that has resulted in the flat lying nature of Caithness. These rocks are termed Caithness Flagstones and were formed in a series of fresh-water lochs during the Devonian geological period, often terms Lake Orcadie. The series of lochs extended over Caithness, Orkney, Shetland, western Norway and the Moray coast.
In northern Scotland the Devonian succession is divided into the Lower, Middle and Upper Old RedSandstone. These divisions roughly translate into the Lower, Middle and Upper Devonian. Often these units are separated by breaks in sedimentation with associated erosion (called unconformities) and represent periods of uplift in the underlying ground level.
In Strathy and Baligill the rocks belong to the Lower and Middle Devonian subdivision and have been termed the Baligill Outlier.
The Lower Devonian was deposited on the irregular basement of post-Caledonian land surface. Beforethese rocks were deposited there was a long period of erosion and weathering. Mountains and hills to the west were cut by rivers and alluvial fans in a fairly hot and arid climate. Flash floods resulted in dumps of sandy material on the (sometimes dried out) lake floor. The area of the Baligill Outlier represents the westernmost margin of the Orcadian lake system.
The Middle Devonian lithologies were deposited in the main period of lake development. The rocks are predominantly grey or green in colour and form a series of cyclical deposits recording deposition in a lake of fluctuating depth and extent.
During deep lake periods dark organic-rich laminated beds were deposited. These contain the fossil remains of boney-headed fish that swam in the lake. The fish beds, as they are termed, are correlated to the fish-bearing layer at Achanarras near Spittal in Caithness.
From the slipway at Port Skerra one can see in the cliff face opposite the relationship between the Devonian rock are the underlying basement. The irregular surface of gneiss is overlain unconformably by conglomerates and sandstones probably representing the deposits of flash floods.
In Baligill, just north of the bridge crossing the stream, a grey carbonate laminite is exposed in the stream bank and is overlain by a asandstone. This is a typical basin margin feature and represents the transition from deep lake to shallow conditions without any transitional lithologies in between. The sandstone fines up into thin bedded flags and a second carbonate laminate sits on top. Both the laminites contain the fossil fragments of the fish that once swam these waters.
Further down stream near the old lime kilns a laminated limestone (thus the lime kilns!) is overlain by a green shales and sandstones. Part of this sequence has been interpreted as having been deposited in moderately deep water by deep currents. The inclusion of granite pebbles indicates that the basement was located nearby. The uppermost sandstone is a shallow water deposit from a river or river-delta origin deposited during a period of low lake level.
At Baligill Quarry two fish-bearing lacustrine laminites are exposed. The organic-rich beds also contain lenses of sand. These were probably rafted into the deep lake environment by being attached to patches of floating algal or bacterial mats. The fauna of the fish–beds contains Gyroptychius, Osteolepis and Palaeospondylus.
Overlying the rocks are deposits that have come from the action of ice, wind and water during the geological period called the Holocene.
Directly above rocks is a veneer of glacial till. This is an accumulation of unsorted, unstratified mixture of clay, sand, gravel and boulders. There are one of two types of till in east Sutherland and Caithness. The first is a shelly till that contains evidence that it was sourced from the floor of the ancient Moray Firth. The second contains no shells, is brown and contains igneous and metamorphic rocks. It is this second till that covers much of the floor of theStrathy valley. Regionally this glacial till extends as far east as Watten in Caithness. There are other deposits related to glaciation that can be found in the area. It is thought that during the ice age there was over 1km of glaciers above the land surface. This ice sheet is thought to have disappeared around 15,000 years ago.
Deposits related to the meltwater and the bulldozing action of the glacier can be found at the margins of the strath. Better examples of these lateral moraines can be found of the eastern side of the Strath of Halladale.
Boulders transported by the glacier and deposited away from their parent lithologies are called erratics. Those that are placed on top of moraines and rock hills are called perched blocks. Many geologists have examined these to try and understand the pattern of glacial movement in a regional context. Examples of these occur on the hills between Reay and Armadale, including the hills above Strathy.
All of these help us to decipher what happened during the last period of glaciation to hit our strath. At that time we can imagine an environment much like Greenland today. Water melts from the glacier and washes through clay, silt, sand, gravel and boulders carrying these further down stream. Larger boulders can be placed upon deposits as the glacier recedes.
As the glaciers receded the weight of the glacier was removed from the landmass. This in turn resulted in the land rising relative to the sea level. The rise in sea level from the melt water was less than the rate at which the land rose - giving the false impression of sea level drop during the Holocene.
Evidence of this land rise can be seen in Strathy too. Just to the west of the mouth of the Strathy river a horizontal, linear feature can be seen on the hill side. This feature is called a raised beach and represents the sea-shore during and just after the last glaciation. This fossil beach is duplicated all along the coastline between Durness and Reay.
As temperatures rose at the start of the Holocene (11,000 year to present) there were significant changes in vegetation. Previously vegetation during the glacial period was limited to Arctic-alpine species and afterwards these moved to higher altitudes. Replacing the alpines were juniper, birch, heather and crowberry. Birch was replaced, based on pollen analysis, by heather at around 6200 years before present.
Throughout the last 11,000 years peat has gradually been accumulating as exemplified along the roadfrom Forsinard to Melvich. Generally the climate has been wet and warm enogh to encourage the growth of bog forming plants, yet cool enough to surpress decomposition. Of course Sphagnum moss is the main constituent of peat bogs; it holds water, produces organic acids that help preserve organic remains and is able to absorb nutrients from water.
At the mouth of the Strathy river there is evidence of coastal deposition and erosion. Deposition isrepresented by the wide long beach that is enjoyed by many people. The shape or outline of the beach is constantly changing in response to the sea state, seasons and wind direction. The wind has also blown the sand into the dune system that we can see today. This too is constantly adapting to the changes in the environment.
Geology represents the main control on the coastal erosion of Strathy Bay. The breaks in the rock, called joints, allow air at high pressure to be compressed by waves into these small gaps. Through time these further split the rocks and create the scenery that we recognise now; gloups, geos and caves.
Above the cliffs the soil clings to the edge. In places horizontal ridges can be seen. These do not represent sheep tracks but slow, gradual soil creep related to soil slope fa