The Area involved is the Solway Firth, one of the largest estuaries in Britain. The onshore area comprises mainly soft low raised beach cliffs, sand dunes and beaches, with sand banks and saltmarsh. Little solid rock is exposed. The Solway Firth is fed by four English rivers, the Eden, Esk, Wampool and Waver, and a number of Scottish rivers including the Sark, Kirtle Water, Annan, Nith, Urr, Dee, Water of Fleet, Cree, and Water of Luce.
Between the English estuaries, the coastline is primarily low saltmarsh formed in the last 10,000 years. However, on the Scottish side there is a greater amount of hard rock. The Solway Firth marks the ancient boundary between two colliding continents.
The violent events of over 400 million years ago created the Southern Uplands of Scotland from the squeezing of the intervening seabed sediments. These dramatic events are recorded on the Scottish shore of the Solway by granites which were intruded at a similar time, but they are largely masked elsewhere by more recent deposits.
These comprise sandstones and mudstones laid down in Triassic deserts some 200 million years ago, and deposits of sands and muds of recent marine origin and tills laid down during the last ice age. These latter deposits are of particular geological importance. The uprising of the land following its release from the weight of the ice has led to the development of a series of fossil raised beaches. The saltmarshes themselves have also been affected by the uplifting land.
They provide the finest examples in Britain of marsh terraces formed by the combined effects of uplift and creek migration. This combination is found nowhere else in Britain and provides excellent evidence for interpreting saltmarsh development in relation to relative sea level changes which have occurred in the last six thousand years.
The saltmarshes also display well developed dendritic creek systems, four types of saltpan and in places a cliff some 2 -3m high. There is a natural arch called Elephant Rock on Hestan Island in the Solway Firth, about a mile and a half from the village of Auchencairn on the Kirkcudbright mainland shore and another at Sandyhills not far down the A710 towards Kippford .
The Needles Eye at Clifton is much nearer to the road down a gentle incline and as such easily accessible. These rocks are Ordovician turbidite muds and silts laid down on the floor of the Iapetus Ocean, which eventually closed with much geological upheaval along the line known as the Iapetus Suture, which runs roughly along the present-day Solway Firth and Southern Uplands.
At first sight, the seemingly vast and intractable turbidite succession of the Southern Uplands of Scotland must have presented a daunting geological challenge. The key that unlocked its secrets was a group of humble fossils called graptolites, the remains of small, free-floating colonies of marine organisms. Through the late Ordovician and the Silurian Periods graptolites were evolving rapidly and so different assemblages of them can be used to date the rocks; at best, graptolites can be used to identify divisions representing the sedimentary accumulation of less than half a million years.
Whilst the late Ordovician seas were encroaching on the subsiding margin of the Laurentian continent, another sedimentary sequence was being deposited further south. Its history is now revealed by the rocks of the Southern Uplands, a geological massif that extends from the North Sea coast of Scotland south-westward into Northern Ireland, spanning the Dumfries and Galloway region. Across this wide expanse of rolling hills are seen tough, resistant sandstones.
They record how, with the resumption of subduction, a deep ocean trench was established into which turbidity flows poured. A characteristic, banded succession of turbidite strata built up, but with important constraints on its development imposed by the highly active tectonic setting, similar to the environment seen today in the earthquake-prone trenches around Indonesia and Japan.
The Southern Uplands turbidite sandstones were deposited on top of the thin layer of muddy sediment that covered the ocean floor. But all the while the ocean floor was very slowly, at a rate of several centimetres per year, creeping towards destruction in the ocean trench subduction zone. So, once deposited, the turbidite sandstones were also carried on towards a similar fate. Instead of disappearing into oblivion, however, successive huge slices of sedimentary rock were stripped off the descending oceanic crust and stacked up, each new slice pushed in beneath those that had gone before.
As the pile of rock slices got bigger, the original, nearly horizontal structures were steepened and now appear almost vertical. The major features seen today originated as the planes of movement, or faults, separating the sequentially incorporated slices. Along coastlines two different types of arches can form depending on the geology.
On discordant coastlines rock types run at 90° to the coast. Wave refraction concentrates the wave energy on the headland, and an arch forms when caves break through the headland, e.g., London Bridge in (Victoria, Australia). When these eventually collapse, they form stacks and stumps. On concordant coastlines rock types run parallel to the coastline, with weak rock (such as shale) protected by stronger rock (such as limestone) the wave action breaks through the strong rock and then erodes the weak rock very quickly.
Good examples of this are at Durdle Door and Stair Hole near Lulworth Cove on the Dorset Jurassic Coast in south England, although these are on an area of concordant coastline.
The Criffel granite complex, to the SW of Dumfries, is the main intrusive body of the area. This area has yielded specimens of romanecheite, amethyst and smoky quartz as hand specimens in recent years. Small crystals of titanite and monazite have also been found. I have it on good authority from The Scottish Natural Heritage that the SSSI status is because of the “HOT” (radioactive) granite rocks identified locally.
Thanks to Stuart Graham SNH and Stephen Blow SWT for their permission to place this cache.
Wikipedia defines - A natural arch or natural bridge is a natural geological formation where a rock arch forms, with an opening underneath. Most natural arches form as a narrow ridge, walled by cliffs, become narrower from erosion, with a softer rock stratum under the cliff-forming stratum gradually eroding out until the rock shelters thus formed meet underneath the ridge, thus forming the arch. Natural arches commonly form where cliffs are subject to erosion from the sea, rivers or weathering (subaerial processes); the processes "find" weaknesses in rocks and work on them, making them larger until they break through. The choice between bridge and arch is somewhat arbitrary.
The Natural Arch and Bridge Society identifies a bridge as a subtype of arch that is primarily water-formed.
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