On the way, you can see

• A major unconformity
• World‑class exposures of Devonian lake sediments
• Fossils and sedimentary structures that illuminate Lake Orcadie’s ecology
• A coastline shaped by extreme Atlantic wave energy

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. The rocks now found on the Stromness coastline were formed during a period which lasted for 5 million years around 390 million years ago. The process was triggered by 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 the islands today.

The West Shore at Stromness provides one of the clearest continuous coastal sections through the Stromness Flagstone Group (Middle Old Red Sandstone, Givetian) within the southern Orcadian Basin. The traverse exposes a representative suite of lacustrine, shoreline, and marginal‑lacustrine facies, The section is structurally simple, with gentle regional dips to the east–southeast, allowing bed‑by‑bed examination of the cyclic lacustrine succession.

Below the Links Battery at Ness, we have the great unconformity that separates the granite of Brinkies Brae from the overlying flagstones. That granite was intruded as magma at depths >4 km into the roots of the Caledonian mountains. Then there was a 40 Myr time gap between that granite cooling to solid rock and the deposition of the overlying ORS. The mountains were eroded down, leaving only the hills of Brinkies Brae, the hill that overlooks the town of Stromness, and a further ancient, low granite hill on Graemsay. The islands were soon submerged and buried as Lake Orcadie lake grew and thick sediments filled the basin.

The overlying rhythmically bedded flagstones comprise millimetre‑ to decimetre‑scale alternations of fine sandstone, siltstone, and laminated mudstone. 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 would dry out completely in an area (playa) and impressions of large salt crystals can be found in the mudstone. Another common feature of drying out of the lake bottom is the formation of large hexagonal “mud cracks”. When the water level is shallow, wave “ripple marks” are formed in sand beds. During a phase of deep water with stagnant muddy bottoms, dark blue-black laminated flagstones are deposited, often containing teeth, scales and bone plates of fish species that had evolved by 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. The cycles represent deepening–shallowing lacustrine couplets, widely interpreted as climatically forced (Milankovitch‑scale) fluctuations in lake level within Lake Orcadie. Dark, organic‑rich mudstones mark deeper, more stable lake phases; thin, sharp‑based sandstones represent shoreline progradation or storm‑generated density underflows during lowstands or regressive pulses. Soft‑sediment deformation structures — including load casts, flame structures, and small slumps — are common, indicating rapid deposition and water‑saturated substrates.

Progressing westward, the succession thickens into shoreface and shoreline sandstones. These beds show low‑angle planar lamination, symmetrical ripples, and occasional trough cross‑sets, consistent with wave‑dominated littoral processes. Localised lenses of well‑rounded granule to pebble gravels occur at erosional bases, representing reworked beach deposits formed during lake‑level lowstands. These facies mark the transition from deep water to littoral environments and record repeated shoreline migration across a broad, low‑gradient lake margin.

A distinctive interval of stromatolitic and peloidal limestones occurs along the mid section of the traverse. These stromatolites(blue‑green algal mats)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 also found. The pale, irregular ledges contain domal and stratiform stromatolites, laminated microbialites, and thin dolomitic interbeds. Their development reflects periodically evaporitic, shallow water conditions, during prolonged lowstands when the lake became hydrologically restricted. Geochemical studies elsewhere in the Stromness succession show enrichment in Fe, Mn, and trace metals within similar horizons, suggesting episodes of elevated salinity and reducing bottom water conditions.

The most readily observed igneous rocks are the various “Orkney Trap Dykes”, the long narrow lines of rock that are the result of basaltic magma 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.

Turning northwards, the cliffs expose a superb continuous section through multiple lacustrine cycles, each typically comprising:

1. Basal erosional sandstone (shoreface or storm generated)

2. Thinly bedded siltstone–sandstone couplets (shallow lacustrine)

3. Laminated mudstone (deep lacustrine)

4. Desiccation features or microbial textures (exposure surfaces)

These cycles form the basis of the classic Orcadian “flagstone rhythmites.” Although fish fossils are not common in situ along this stretch, the lithofacies correspond to the productive horizons elsewhere in the Stromness and Rousay Flagstone Groups, which yield placoderms, acanthodians, and early sarcopterygians.

The modern coastal geomorphology is strongly controlled by the orthogonal joint sets and the mechanical anisotropy of the flagstones. Persistent Atlantic swell exploits bedding planes to form broad shore platforms, while vertical joints guide the development of geos, clefts, and incipient sea caves. Storm events regularly transport large sandstone blocks onto the upper foreshore, demonstrating the extreme wave energy characteristic of the western Orkney archipelago.