Seabed and coastal geology and sediments

Status: Purely descriptive 

Geology and geological processes form the foundations for why the Solway Firth exists in its current form. Geology has impacted the ecosystems which have developed therein, the industries which can thrive, and the communities which settle in the surrounding area.

Over 400 million years ago (Silurian Period) there were two continents called Laurentia and Avalonia. These continents were located to the North and South, respectively, of the Lapetus Ocean. The land we now know as Scotland formed part of Laurentia, and England, Avalonia. Over the course of millions of years these continents moved closer together. Eventually, these two land masses collided, contributing towards the creation of Pangea by 250 million years ago. The silts and sands on the ocean floor of Laurentia folded into sandstones and formed the Southern Uplands of Scotland. During this process, superheated molten rock, or magma, rose and cooled, producing granite mounds under the Earths crust. As surrounding softer rocks weathered away these granite hills became exposed and today they are known as Criffel and Screel Hill on the Scottish Solway shore. The origins of the Scotland and England, as part of two different continents, is the  reason for the vast differences in geology seen in two countries geographically close in the present.

340 million years ago (Carboniferous period) the Solway was near the equator and repeatedly cycled through shallow seas and swampy forests. The remains which are preserved today as coal seams resulted from the compression of vegetation by sediments. The sand and mud from the seas/forests allowed limestone beds to develop.

260 million years ago (Permian period) the Solway was an arid desert, forming red sandstone. This is still quarried in Dumfries and Galloway today and is a frequently seen in local features and buildings.


Permian Rocks at Loch Ryan. © N. Coombey

Permian Rocks at Loch Ryan. © N. Coombey

210 million years ago (Triassic period) the Solway was still hot with seasonal monsoons that washed sand into layers and then cemented to create sandstone.

Over the last 1.6 million years, the Solway was reshaped into what we see today. Glacial periods scoured valley bottoms and exposed layers of rock. The weight of ice during the last ice age caused land to sink. Now that the ice has melted some land is now rising slightly due to gradual post-glacial rebound. The Solway is one of these areas, offsetting a portion of sea level rise. This rebound will lessen over time, eventually stopping entirely.

To this day the ‘Solway Line’ is the geological boundary which separates Scotland and England and marks the fault where these once separate continents joined. Maryport also features a fault line between Triassic rocks and older Carboniferous rocks with coal containing outcrops.

Readers may also be interested in the Solway Review’s section on Intertidal Rock, sediments and biodiversity.


Image; Coral Fossil at Powillimount. © G. Reid/ Solway Firth Partnership

Seabed and coastal geology and sediments

Solid Geology

Bedrock geology is the underlying foundational rock of the Earth and can either be exposed to the surface, or have deposits covering it.

The Solway Firth occupies a tapering trough formed during the Carboniferous period against the older Ordovician and Silurian rock of the Scottish Southern Uplands, overlaid by more recent sedimentary deposits. As such it creates a convenient exit point for various rivers, notably the Eden and Esk, although it should not be regarded merely as a river estuary.

One of its most noticeable features is the dramatic difference in character between the facing coastlines. The low lying “dune” nature of much of the Cumbrian coast, gradually rising to higher ground and ultimately the high core rock of the Lakeland Fells is sharply contrasted by the more rugged glacially cut hard rock of the Scottish Coast west of Dumfries with its characteristic north-south aligned bays and inlets between promontories and headlands featuring high cliffs which guard sandy bays and saltmarshes.


Extensive parts of the Solway Basin overlie Carboniferous rocks including Westphalian Coal Measures. The deep burial of these rocks has led to the generation of gas and oil. The hydrocarbon compounds from the coal measures have migrated upwards into the overlying Triassic sandstones and, where these have become trapped, have produced economic accumulations of gas (e.g. Morecambe Bay).

During the last Pleistocene glaciation one ice sheet flowed down the western Irish Sea and spilt over eastwards into the shallower, eastern part. This ice sheet interlaced with the ice sheets flowing off the surrounding hill masses to deposit a complex sequence of sediments. It has been suggested that the scars or ridges of boulders which occur in parts of the Firth are the marine equivalents of drumlins, from which the fine sediments have been winnowed by waves or tidal currents.

Glacial drift has been deposited over much of the floor of the Solway Firth and it has been recently shown that these in turn lie on top of glacial boulder clay, or till. These sediments may be several metres or even tens of metres thick, and are presumed to be the major source of material for the developing sandflats and merses of the inner Solway. Marine sediments in turn lie on top of the glacial drift, with increasing thickness towards the head of the Firth. Surface sediments are mainly gravely in the outer part of the Firth and sandy in the inner part.


Scottish Coast

On the Scottish side of the Solway the geology consists of a variety of Granite, Limestone, Graywacke, Lower Coal Measures, Ordovician – Silurian, Lias, Permian Sandstone, and Triassic Sandstone.

Silurian and Ordovician strata predominate from the Mull of Galloway to to Abbey Head, near Dundrennan. From here this strata is cut off from the sea by the lower Carboniferous as far east as Cummertrees. More recent deposits of Permian sandstone overlie these, forming the level areas at the head of Luce Bay, and Lochar Moss south-east of Dumfries. The remaining coast inland to Carlisle is characterised by the low, gently rolling landscape overlying triassic sandstone.

Dramatic intrusions of volcanic origin dominate the area south-west of Dumfries to form the high granite slopes of Criffel, Long Fell, Maidenpap and Screel Hill south of Castle Douglas.

A further, isolated example is to be found in the far west at Crammag Head beyond the Mull of Galloway.

The Management Strategies for the National Scenic Areas on the Dumfries and Galloway coast (Fleet Valley, East Stewartry, Nith Estuary) each provide an overview of the geology and drift geology of the respective area. The Management Strategies are available here.


English Coast

On the English side of the Solway there is a variety of Middle and Lower Coal Measures, Milstone Grit, Permian Mudstone, Triassic Sandstone, and Triassic Mudstone.

Along the inner Solway Estuary both the English and Scottish shores appear similar in character, exhibiting low shores and wide mud flats at low-tide.

The underlying rock of both sides of the inner Solway are of the same Triassic era but with the younger mudstone sediment giving rise to the low lying mosses of Burgh-by-Sands.

These mudstones continue south as far as Dubmill Point where the Triassic Sandstone of the Annan area re-occurs round the whole sweep of Allonby Bay to Maryport. The sandstone has been eroded away by glacial action to reveal the older rocks of the middle coal measures giving rise to the heavily industrialised area from Maryport to Whitehaven. These strata have been deformed and displaced by volcanic activity within the central lake district to form, in simplistic terms, an inverted basin or dome which has undergone modification by various glacial events. This has led these middle coal measures to be scoured away, toward their mid point at Workington, to reveal outcropping lower coal measures. This coal bearing strata is composed of successive layers of coal, shale, sandstone bands and grey mudstones of varying thickness, all heavily faulted.

The coastline south of Whitehaven logically mirrors that north of Maryport by the re-occurrence of the Triassic sandstones but at a greater elevation above present sea level. The characteristic up to 80 metre red rock cliffs of Saltom Bay and St Bees Head mark the southern boundary of the Solway area covered by Solway Firth Partnership. Around St Bees Head this is known as ‘St Bees New Red Sandstone‘ according to the geology section of, and the cliffs provide an excellent location for seabirds (See Protected Areas, and Birds sections) and also provides an excellent location to look at a cross section of the rock lying below the surface we see from day-to-day.


Solway Underlying Solid Geology

The Scottish National Marine Plan Interactive provides a shape file of bedrock geology for the marine area for the whole of the UK, as seen in the image opposite. To provide an overview of the variety of underlying geology of the Solway Firth, below are some examples of the types of rocks, time periods, and groups, which make up the Solway bedrock;

Rock types include;

  • Sandstone
  • Mudstone
  • Wacke
  • Felsite
  • Limestone
  • Granitic Rock
  • Halite Stone

Time periods include;

  • Ordovician
  • Silurian
  • Carboniferous
  • Permian
  • Lower Permian
  • Triassic
  • Lower Jurassic
  • Unknown Ages – Igneous intrusions

Groups and formations include;

  • Carboniferous Limestone Supergroup
  • Sherwood Sandstone Group
  • Kirklinton Sandstone Formation
  • St Bees Sandstone Formation
  • Coal Measures Group (England)


Image; Scotland’s National Marine Plan Interactive, with layers (links will provide usage licence, data provider, etc); ‘Solway Region (mask)‘ © Crown Copyright, All rights reserved, ‘BGS Offshore 1:250,000 scale marine bedrock polygons (BGS WMS)‘ © NERC. All rights Reserved, and ‘Limits and Boundaries – NE and NW Marine Plan Areas for England (MMO) – August 2016‘ © Crown Copyright, All rights reserved.
The opacity of certain layers has been changed in order to try and show both the limits of the Scottish side of the Solway, and the English side of the Solway (as part of the North West Marine Plan inshore area), in relation to the marine bedrock layer. Please note that the colours in the marine bedrock layer in this image are distorted due to the colouring of the North West Marine Plan inshore area layer.


Seabed and coastal geology and sediments


British Geological Survey have created a series of geological maps for Scotland and also a series for England and Wales. These maps are a collection of geological maps of either scale 1:50,000 or 1:63,360 and vary in year of publication and also type or types of geology they depict.

These maps are highly relevant for terrestrial geology but have limited information about the marine geology underlying the water of the Solway. Some provide limited information relating to the coastal geology. For example, the 1966 solid and drift geological map for Stranraer within the Scottish series provides some data on coastal deposits of sand & gravel and boulder clay as well as margins of fluvial-glacial terraces, beaches and raised beaches.

The most relevant and useful maps in discussing the geology of the Solway Firth within these series’ are both located within the Scottish series and are;

Area Type of Geology Hyperlink
Solway East Superficial with Simplified Bedrock Map
Solway West Superficial with Simplified Bedrock Map

These maps depict deposits in the inner firth area, and the underlying geology which is below these deposits.

Looking at the ‘Solway East’ map, you can see that the Scottish side of the inner solway is primarily made up of raised tidal flat deposits along to Annan in the east, with some salt marsh present from this point along to Gretna. Other deposits seen in this area of the coast include raised beach deposits, with marine beach deposits at Barnkirk Point and Powfoot, and are also from the Flandrian interglacial stage of the Quartenary period. The intertidal area consists of a variety of salt marshes, intertidal sand flat deposits, and intertidal sandbank deposits.

These ‘Scottish’ maps also provide geological information on the Cumbrian side of the Solway. Looking at the ‘Solway East’ map, which extends from the Scottish border west to roughly Seaville, you can see artificially induced alluvial deposits, raised tidal flat deposits, till, and raised marine beach deposits make up most of the Cumbrian Solway coastal deposits. The intertidal area of the Solway, similarly to the Scottish side, is primarily salt marsh and intertidal sand flat deposits.

The ‘Solway West’ map has limited information about the English side of the Solway’s coastal geology, however it does provide for Grune Point to Wolsty. Storm beach deposits, marine beach deposits, and blown sand are the primary coastal deposits on this stretch of coastline. The intertidal area, similarly to the area discussed above, is primarily made up of intertidal sand flat deposits, with Stenor Scar and Grune Point being highlighted as Diamicton, and Stenor Scar being from the Mercia Mudstone Group.


Detailed information about excursions centred around geology around parts of the Scottish Solway is available via Earthwise, British Geological Survey;


Image; Vertical strata at Monreith. © N. Coombey/ Solway Firth Partnership.

Seabed and coastal geology and sediments

Cultural Geological Features

Much like other features around the Solway, geological features have long standing stories attached to them. Unique geological features form part of history through place names, location markers, and form an important part of traditions and folklore often providing the focal point or element the story is built around. Many of these stories are still passed down today or are the origins of place names forgotten over time. With many of these features around the Solway the following are just two examples of how geology remains an important part of the heritage and culture today, for example; the Devil’s Stane, and the Altar Stone. For more information please see Solway Firth Partnership’s Coastwise Project (concluded 2019), which sought to find the origins of place names on the Scottish side. Place name origins are often connected to the geological features of an area, such as Rough Island/Firth, where the granite outcrops form a rugged coastline, and is likely the reason behind the origin of ‘Rough’ meaning uneven. Lot’s Wife is another example. This Silurian sea stack, surrounded by cliffs of Silurian greywackes with porphyritic intrusions originates from the biblical story where Lot’s wife is turned into a pillar of salt.


Devil’s Stane

The Devil’s Stane sits on the coast at Powillimount, with little explanation for why this granite boulder is located here. Legend says that the Devil took a bite out of Criffel and, disliking the taste, spat out this chunk. Supposed proof of this story lies in the ‘teeth marks’ which can be seen on the top of the stone. These marks are actually abandoned work to split the rock.


Altar Stone

The Altar Stone is a boulder on the sands south of Annan that has been a boundary marker since medieval times. The stone marks the parish and Royal Burgh boundary as well as defining fishing rights. It is also the target mark for riders in Annan’s Riding of the Marches, when tides allow for this. The Altar Stone is not always visible, at times covered by the shifting sands of the Solway, but its position is still used for these purposes.


Altar Stone. © N. Coombey

Altar Stone. © N. Coombey

Image Opposite; Devil’s Stone near Carsethorn. © G. Reid/ Solway Firth Partnership

Seabed and coastal geology and sediments


Climate change, which is producing warmer temperatures, may increase loss of organic matter from soils, contribute to greater coastal erosion from increased rainfall and sea level rise. In turn this may alter geology in terms of eroding rocks or deposition.

As previously mentioned, geology can have a huge impact on settlement patterns, industrial, and agricultural activities. The Solway has traditionally been exploited for coal on the English side as extensive parts of the Solway Basin overlay Carboniferous rocks including Westphalian Coal Measures. The deep burial of these rocks has led to the generation of gas and oil. Various mining and quarrying activities have been undertaken on the Solway throughout history. There was also a gypsum and alabaster mine at Saltom Bay. The ‘Barrowmouth mine’ is now a scheduled monument through Historic England, for more information on this mine click here. Other valuable formations are typically extracted, such as granite or sand. However, the results from this process can cause environmental impacts such as increasing air and noise pollution, among others. The past has seen lucrative mining of various materials surrounding the Solway coast, such as coal, sand, gravel, and granite. Sand and gravel mining continue to take place around the Solway, for example in Sandhead, Dunragit, and Hoddam Contracting’s Broom Quarry, near Powfoot.


Granite Quarry near Creetown. © N. Coombey

Granite Quarry near Creetown. © N. Coombey

The mining of metals such as lead, zinc and copper deposits has been relatively common throughout Dumfries and Galloway, as illustrated by the pink dots on the map opposite. Although extensive mining activities occurred at Leadhills (just outside of Dumfries and Galloway) inland of the Solway, several mines operated in areas closer to the Firth, such as near Gatehouse of Fleet. In 1770, chalcopyrite, malachite and azurite were reportedly present and being mined at a coastal location near Colvend, while Baryte mines at Barlocco (producing ~3000 tonnes) and Auchencain (producing ~700 tonnes) were in operation between 1856 and 1920, and in the 1860s respectively (Stone et al, 2012).

Uraninite is also present and has been the topic of studies at the Piper’s Cove/Needle’s Eye, south-west of Caulkerbush.


Image; Stone et al (2012) image P912376, available here.

Seabed and coastal geology and sediments


Baxter, J.M., Boyd, I.L., Cox, M., Donald, A.E., Malcolm, S.J., Miles, H., Miller, B., Moffat, C.F., (Editors), (2011). Scotland’s Marine Atlas: Information for the national marine plan. Marine Scotland, Edinburgh. pp 191. Available here. (Accessed 22.07.19)

British Geological Survey (2011). The Quaternary Geology of the Solway, Geology and Landscape (Scotland) Programme Research Report RR/11/04.  Available here. (Accessed: 11.10.19)

Cameron D. G., Bide T., Parry, S. F., Parker, A. S., and Mankelow, J. M. (2014) Directory of Mines and Quarries, 10th Edition. Available here. (Accessed: 14.10.19)

Cumbria Biodiversity Partnership (2001). The Cumbria Biodiversity Action Plan. Available here. (Accessed: 29.07.19)

Dumfries and Galloway Biodiversity Partnership (2009). Dumfries and Galloway Local Biodiversity Action Plan. Available here. (Accessed: 29.07.19)

Joint Nature Conservation Committee. (n.d.). The Geological Conservation Review Database. Available here. (Accessed: 22.05.19)

Marine Management Organisation. (n.d.). Marine Planning Evidence Base. Available here. (Accessed: 14.05.18)

Marine Scotland (n.d.). Scotland’s National Marine Plan Interactive. Available here. (Accessed: 06.08.19)

McMillan, A., & Stone, P. (2008). Southwest Scotland: a landscape fashioned by geology. NatureScot. Available here. (Accessed: 11.10.19)

Mills, F., Sheridan, S. and Brown S., (2017). Clyde Marine Region Assessment. Clyde Marine Planning Partnership. pp 231. Available here. (Accessed: 14.05.18)

National Coastal Change Assessment. (n.d.). Available here. (Accessed: 12.07.19)

Ramsay, D. L. & Brampton, A.H. (2000). Coastal Cells in Scotland: Cell 7- Mull of Galloway to the Inner Solway Firth. NatureScot, Research Survey and Monitoring Report No 149. Available here. (Accessed: 08.08.19)

Scottish Geology Website. (n.d.). Available here. (Accessed: 17.02.18)

Solway Firth Partnership (1996). The Solway Firth Review, Dumfries. Available here. (Accessed 23.07.19)


In-Text References; 

British Geological Survey (n.d.). Geological Survey of Scotland, 1:63,360/1:50,000 geological map series. Available here. (Accessed: 08.10.19)

British Geological Survey (n.d.). Geological Survey of England and Wales 1:63,360/ 1:50,000 geological map series, New Series. Available here. (Accessed 10.10.19)

Stone, P., McMillan, A. A., Floyd, J. D., Barnes, R. P., & Phillips, E. R. (2012). British Regional Geology: South of Scotland 4th ed. British Geological Survey. Available here. (Accessed: 22.05.19)


Image; Mossyard. © N. Coombey/ Solway Firth Partnership.