Geological Field Trip - West Shore, Stromness.
Sedimentary Rocks Formed at the Margin of "Lake Orcadie"
It is obviously not possible to take in the complete geology of Orkney within the scope of a short field
excursion, but the rocks along the West Shore of Stromness are remarkable for the quantity of
information you can get from a short stretch of rocky coastline.
The rocks of the Orkney coastline were formed during a period which lasted for 5 million years (written
5my), and which started 390 million years ago. What started the process off was the gradual sinking of
the land surface in the area between Orkney and Norway and the filling of the resultant depression with
over 3500 meters (11,500 feet) of lake and desert sediments. Over time, these lake and desert sediments
became rocks, which we can see in the quarries and cliffs of these islands today.
This excursion will demonstrate many of the general principles and processes that are required to enable
you to read the geological record, the story recorded in the rocks. The most basic principle of all is this:
The present is the key to the past. In other words, by studying the way that natural processes operate
today, such as the weathering of rocks, the build-up of sandy beaches and so on, you learn to recognise
the signs of these processes in operation; you can then look out for similar signs in the layers of rock and
interpret what happened when these layers were laid down.
On this outing, we can see the present-day beach environment and note its features. Then, at a site below
the "Gun Emplacement" at Ness, we have the remains of an ancient beach, fossilised in the rocks. In the
ancient Lake Orcadie, there was a granite island: it is still present today as the hill behind the town of
Stromness, with a further outcrop of the granite on the north coast of Graemsay.
Today you can see the processes of erosion of rocks, transportation of sediments, and deposition of
sediments in action on a beach such as this. The pictures of these processes on a local scale can be made
use of to help to envisage the setting of Lake Orcadie on a regional scale as outlined now in a brief
geological history.
It is now believed that the continents of the earth are not absolutely fixed and rooted to the earth's
surface, but that instead they can move or drift, very slowly, on the underlying material. You can see on
a map of the earth that some continents protrude and others have hollows, so that it is possible to think of
the continents as at one far-off time fitting together like a jigsaw, and then gradually drifting. The theory
is that the present-day continents did not separate out at the start of the break-up of this original
continental mass, but rather that one or two continental masses broke away first, and then themselves
split, to give the present-day shapes of continents.
Between 425 my and 405 my ago, two of these large continental masses collided. These were Laurentia,
which was moving eastwards, and Baltica, which was going to the west. They collided along a zone that
runs from Ireland through Scotland, West Norway, Orkney and Shetland, to North-east Greenland, and
where the colliding edges met, a high mountain chain emerged. (Elsewhere on the earth, the same
process, along another collision boundary between drifting continents formed the Himalayan mountain
chain.)
The two continental masses, which collided in this way, were thereby welded together into what is
known as a super-continent, which is called Laurasia. (By the way, the study of these movements of
continents like huge plates floating on an underlying material is called plate tectonics.) The formation of
the super-continent of Laurasia left the area on which modern Orkney was to form isolated from the
oceans in a mid-continent position.
As time went on, these new mountains were themselves worn down by the physical processes of frost,
wind, rain and rivers. As the mountains were worn down, their cores of granite and metamorphic rocks
were exposed. (Recall that metamorphic rocks are simply rocks, whether sedimentary or igneous in
origin that have been squeezed and cooked by great pressure and heat within the earth, so that they
change their entire structure, or metamorphose.) These rocks of the ancient mountain chain, that were
once deep in the earth's crust and were gradually exposed, are commonly observed today in the west
Highlands of Scotland - but also in the Stromness area. This is the origin of the granitic island that today
is the hill behind Stromness and which once was washed by the waters of Lake Orcadie. The formation
of the Lake was preceded by the formation of the Orcadian Basin.
During the period of erosion, when these deep crustal rocks were gradually emerging, approximately
400 my ago, other massive geological events were also taking place. Large fractures or faults were
appearing in the earth's crust, such as the Great Glen Fault that lies 20 km to the east of Orkney and
passes through central Shetland). These faults formed low-lying basins to the east of Orkney, with high
hills to the west. Continued erosion of these mountains filled the low lying basins with thick deposits of
sands - this was to become the Old Red Sandstone.
By the stage known as Middle Devonian times (the geological time period which includes the time-span
when the Orkney sedimentary rocks were formed, 390 my ago), the region occupied by Orkney was a
low-relief mid-continental desert, similar to the present-day deserts of Utah and Arizona in the USA,
with the open ocean located in the south near the present coastline of the English Channel.
Rainfall in the hills surrounding the "Orcadian Basin" collected in large river systems that flowed into
the continually sinking Orcadian basin, bringing floods of sand and forming the large mid-continent
Lake Orcadie that stretched from the Moray Firth across to West Norway, back to North-west Shetland
and south through Orkney and Caithness.
During the Middle Devonian period, Lake Orcadie was located south of the equator at a latitude between
20 S and 15 S. This ancient latitude, or palaeolatitude, controlled the climate, a dominant feature being
continued evaporation from the surface of the lake. Evidence of this seasonal drying out can be observed
on many of the bedding planes of the lake sediments.
By late Devonian times, approximately 380 my ago, the area of Lake Orcadie that is in the vicinity of
modern Orkney had completely dried out and sand dunes swept across the area. These thick desert sand
dunes are preserved in the Hoy Hills and the Eday sandstones, which were used to build St Magnus
Cathedral.
The lake and desert deposits of the Orkney area continued to be buried under younger sedimentary
formations for the following 20 my, at which point in time mountain-building processes in Central
Europe caused the gradual uplift of the Orkney rocks until they reached the surface in the recent past.
The present landscape of Orkney is related to the passage of the Scandinavian ice sheets in a
northwesterly direction, smoothing out the pre-existing landscape. When this ice retreated 10,000 years
ago, rock pebbles, sand and clay from the melting ice were left behind as “boulder clay", to be found
mainly in the low lying ground with thicknesses of 3 meters to 10 meters being deposited. Local glaciers
continued to exist after the retreat of the ice sheet, and formed the corries and glaciated valleys of Hoy
and Rackwick.
Material observed in the boulder clay (particularly in the east of Orkney) includes far-traveled "erratic"
pebbles of igneous rocks from the Oslo area and chalk flints picked up from the central parts of the
North Sea. The contribution of these local flints to the post-Ice Age cultures developed in the islands
5000 years ago has never been assessed.
At the present time, the evidence suggests that Orkney is gradually undergoing submergence. Evidenced
for this is the presence in many locations of peat banks found below the high-water mark in sheltered
bays and the existence of freshwater lochs at sea level, separated from the sea by gyres or accumulations
of "blown sand".
The West Shore – Stromness
Islands such as the "Stromness Island" are common near the margins of the Orcadian basin, in the lower
part of the "sedimentary succession". Such islands were soon submerged as the lake grew and thick
sediments filled the basin.
The depth of water in the lake varied gradually through time from shallow to deep, then back to shallow.
Since different sedimentary rock types are formed in different water depths, it is possible to observe
these cycles on the Stromness shore. Occasionally the lake will dry out completely in an area and
impressions of large salt crystals can be found in the mud. Another common feature of drying out of the
lake bottom is the formation of large hexagonal "mud cracks". When the water level is shallow, "ripple
marks" are formed. During a phase of deep water with stagnant muddy bottoms, dark blue-black
laminated flagstones are formed, often containing teeth, scales and bone plates of fish species that
evolved at this time. Fossil fish are abundant in the dark-coloured flagstones of all the Orcadian rock
sequences, but are best observed in the thick Sandwick and Eday Fish Beds.
Besides the fossil fish remains, the flagstones contain abundant "stromatolite" fossils, which take on
several different forms. These stromatolites are the structures created by algal growths near the lake
margins, and several excellent examples are to be seen directly below the old "Bathing Shelter" at Ness,
where large algal mounds have been preserved. Rare fossil plants can be found in association with the
fish beds, and occasionally minute bivalve shells are found.
Although not scheduled for this excursion, the protected locality for the "Horse Tooth Stone" on the cliff
face 100 meters north of the parking stance at Yesnaby is another excellent example of stromatolite
structures.
At each stage of erosion, there was sand and sediment which had been formed out of the grinding away
of the surface rocks, and so the sand and sediment varied from one period to another. The metamorphic
rocks of the Stromness Island contained a number of heavy minerals, and so produced beach sands with
these minerals in the grains. The action of the sea in shaking and turning these heavier grains eventually
concentrated them into "placer" deposits. Among the heavy minerals that went through this sequence of
processes are uranium minerals. The Stromness placer deposit of uranium is rich enough for it to have
been considered in the 1970s as a low-grade ore, but it is nowhere near the quality of other economic
sources. It is estimated that it would cost ten times as much to extract this quality of uranium as it would
in better areas.
Similar economic restrictions (with also smallness of deposit) make the old Boltifer lead mine at
Warebeth a curiosity rather than an economic proposition. At the same time, the vein mineralisation
contains 77 oz. of silver per ton of ore as well as lead and zinc. The mine was worked for a short time in
1775 for the lead content of the ore.
Igneous rocks of Devonian age are not common in Orkney. Lava flows can be observed at the base of
the Kame of Hoy and at the Point of Ayre in Deerness. Volcanic ash deposits are prominent in the cliffs
at the west end of Rackwick beach. Volcanic vents and pipes (the core of a volcano) can be seen at the
Round Hill in Hoy, at Melsetter, and in Deerness.
For the casual observer of the Orkney shoreline, the most readily observed igneous rocks are the various
"Orkney Trap Dykes", the long narrow lines of rock that are the result of lava intruding into cracks and
faults of the earlier sedimentary rocks. The trap dykes are easily recognised by their black colour and the
fact that being only approximately 1 meter wide, they run generally in an east-west direction directly
across the normal rock layers. They are normally harder than the surrounding sediments, and therefore
stand upright as the softer rock around erodes, so that they form a natural wall or dyke. These rocks are
much younger than the sediments into which they intrude - about 100 my younger. The trap dykes are
probably linked in time to the formation of the present North Sea basins that contain the majority of the
UK oil deposits. A good example of such a trap dyke will be seen at the "Staigg of Nethertoon" to the
east of Stromness Churchyard.
Further reading
British Regional Geology - Orkney and Shetland by W. Mykura (HMSO, 1976).
Geological maps
1 inch to 4 miles: Sheet 3, "Orkney".
1 inch to 1 mile: 117 (Hoy), 118 (Copinsay), 119 (Kirkwall), 120 (Stronsay), 121 (Westray), 122
(Sanday).
The West Shore of Stromness shows clearly
the fossil shoreline where shallow lake
waters contact the granite island of
Stromness. The 'flagstones' formed in the
lake show many features of shallow water
and periodic drying out, i.e. ripple marks and
mud cracks. Fossil fish remains and algal
stromatolites are common.
Middle Devonian Orkney was located at
the western margin of a large lake 15 ° to
20° south of the equator, surrounded by
desert sands and charged with water and
sediments from rivers flowing from the
Laurentian Continent in the NW.
Since the last Ice Age, evidence shows that Orkney
has been sinking below the sea. Flat areas at 35-50
m and at 20 m are noted on marine charts. At 50
m Orkney is one large island, at 20 m it becomes
two islands. It would be interesting to know if the
archaeologists see evidence of different cultures
between the North Island and the South Island?