Dob's Linn

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Dob's Linn
Looking down Dob's Linn from the Top - geograph.org.uk - 1093267.jpg
Looking down Dob's Linn Gorge from the top
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Dob's Linn
Floor elevation300 m (980 ft)
Geography
CountryScotland
DistrictDumfries and Galloway
Coordinates 55°25′47″N3°16′11″W / 55.4297°N 3.2697°W / 55.4297; -3.2697  

Dob's Linn is a small steep valley in Dumfries and Galloway, just north of the A708 road between Moffat and Selkirk, in Scotland. It is part of the Grey Mare's Tail Nature Reserve which is owned by the National Trust for Scotland. [1] According to tradition, Dob's Linn is named for a covenanter, Halbert Dobson, who took refuge there from Government troops during The Killing Time in the late 17th century. [2]

Contents

Dob's Linn is important in geology as the location of the Global Boundary Stratotype Section and Point (GSSP) [3] which marks the boundary between the Ordovician and Silurian periods, and marks the base of the Llandovery epoch, on the geologic time scale. Dob's Linn was ratified as the GSSP by the International Union of Geological Sciences in 1984. [4]

The boundary is defined as the first appearance of graptolites Parakidograptus acuminatus and Akidograptus ascensu 1.6 m above the base of the Birkhill Shale Formation. The shale section also contains chitinozoa and conodonts, but neither are well preserved. Dob's Linn has been criticized for the difficulty in relating its graptolite biostratigraphic sequence with shallow water sequences elsewhere, although the stratotype also appears to correspond with a carbon-13 isotope excursion in the latest Ordovician which can be identified worldwide. [5]

The area was first studied by Charles Lapworth in the late 19th century. His work established fossil graptolites as a method of understanding stratigraphic sequences. [6] A more recent description of the area, enumerating the zones established by Lapworth, is given in the British Regional Geology Monograph. [7] Before Lapworth's work, it was thought that the Silurian rocks of the Southern Uplands formed a single sequence, that would have to be about 6000m in thickness. By his identification of particular graptolite species in different zones of the Dob's Linn exposure, Lapworth was able to demonstrate that the Uplands consist of a much thinner layer, consistent with Silurian deposits elsewhere, that had been repeatedly folded and faulted, with multiple repetitions of the same strata, often upside down. [8] The understanding that sequences of sedimentary rocks could be inverted played an important part in the later resolution of the Highlands Controversy in which Lapworth was also involved.

Related Research Articles

The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period 485.4 million years ago (Mya) to the start of the Silurian Period 443.8 Mya.

Silurian Third period of the Paleozoic Era 444-419 million years ago

The Silurian is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.8 million years ago (Mya), to the beginning of the Devonian Period, 419.2 Mya. The Silurian is the shortest period of the Paleozoic Era. As with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by a few million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when up to 60% of marine genera were wiped out.

Charles Lapworth

Charles Lapworth FRS FGS was a headteacher and an English geologist who pioneered faunal analysis using index fossils and identified the Ordovician period.

Graptolithina Subclass of Pterobranchia in the phylum Hemichordata

Graptolithina is a subclass of the class Pterobranchia, the members of which are known as graptolites. These organisms are colonial animals known chiefly as fossils from the Middle Cambrian through the Lower Carboniferous (Mississippian). A possible early graptolite, Chaunograptus, is known from the Middle Cambrian. One analysis suggests that the pterobranch Rhabdopleura represents extant graptolites. Studies on the tubarium of fossil and living graptolites showed similarities in the basic fusellar construction and it is considered that the group most probably evolved from a Rhabdopleura-like ancestor.

Ordovician–Silurian extinction events Mass extinction event at the end of the Ordovician period and the beginning of the Silurian period in the Paleozoic era, around 444 million years ago

The Ordovician–Silurian extinction events, also known as the Late Ordovician mass extinction, are collectively the second-largest of the five major extinction events in Earth's history in terms of percentage of genera that became extinct. Extinction was global during this period, eliminating 49–60% of marine genera and nearly 85% of marine species. Only the Permian-Triassic mass extinction exceeds the Late Ordovician mass extinction in biodiversity loss. The extinction event abruptly affected all major taxonomic groups and caused the disappearance of one third of all brachiopod and bryozoan families, as well as numerous groups of conodonts, trilobites, echinoderms, corals, bivalves, and graptolites. This extinction was the first of the "big five" Phanerozoic mass extinction events and was the first to significantly affect animal-based communities. However, the Late Ordovician mass extinction did not produce major changes to ecosystem structures compared to other mass extinctions, nor did it lead to any particular morphological innovations. Diversity gradually recovered to pre-extinction levels over the first 5 million years of the Silurian period.

Dumfriesshire Historic county in Scotland

Dumfriesshire or the County of Dumfries is a historic county, registration county and lieutenancy area of southern Scotland.

The Hirnantian is the final internationally recognized stage of the Ordovician Period of the Paleozoic Era. It was of short duration, lasting about 1.4 million years, from 445.2 to 443.8 Ma. The early part of the Hirnantian was characterized by cold temperatures, major glaciation, and a severe drop in sea level. In the latter part of the Hirnantian, temperatures rose, the glaciers melted, and sea level returned to the same or to a slightly higher level than it had been prior to the glaciation.

The Tremadocian is the lowest stage of Ordovician. Together with the later Floian Stage it forms the Lower Ordovician Epoch. The Tremadocian lasted from 485.4 to 477.7 million years ago. The base of the Tremadocian is defined as the first appearance of the conodont species Iapetognathus fluctivagus at the Global Boundary Stratotype Section and Point (GSSP) section on Newfoundland.

The Furongian is the fourth and final epoch and series of the Cambrian. It lasted from 497 to 485.4 million years ago. It succeeds the Miaolingian series of the Cambrian and precedes the Lower Ordovician Tremadocian Stage. It is subdivided into three stages: the Paibian, Jiangshanian and the unnamed 10th stage of the Cambrian.

The Darriwilian is the upper stage of the Middle Ordovician. It is preceded by the Dapingian and succeeded by the Upper Ordovician Sandbian Stage. The lower boundary of the Darriwilian is defined as the first appearance of the graptolite species Undulograptus austrodentatus around 467.3 million years ago. It lasted for about 8.9 million years until the beginning of the Sandbian around 458.4 million years ago.

Paibi Village in Hunan, Peoples Republic of China

Paibi, a village in Paibi Township, Huayuan County, Xiangxi Tujia and Miao Autonomous Prefecture Hunan, China, is the location of the Global Boundary Stratotype Section and Point (GSSP) which marks the boundary between the third and Furongian epochs of the Cambrian Period on the geologic time scale. Paibi was selected over the Kyrshabakty River section, Malyi Karatau, Kazakhstan, and the GSSP was ratified by the International Union of Geological Sciences in late 2003. It established the first formally agreed upon subdivision of the Cambrian. The village gives its name to the Paibian Age in the Cambrian Period.

The Diabasbrottet Quarry, located on Mt. Hunneberg, Västergötland, Sweden, is the location of the Global Boundary Stratotype Section and Point (GSSP) which marks the lower boundary of the Floian stage of the Lower Ordovician.

Sularp Brook, Lund Municipality, Skåne, Sweden, is the location of the Global Boundary Stratotype Section and Point (GSSP) which marks the boundary between the Middle and Late epochs of the Ordovician period on the geologic time scale.

In the geologic timescale, the Rhuddanian is the first age of the Silurian Period and of the Llandovery Epoch. The Silurian is in the Paleozoic Era of the Phanerozoic Eon. The Rhuddanian Age began 443.8 ± 1.5 Ma and ended 440.8 ± 1.2 Ma. It succeeds the Himantian Age and precedes the Aeronian Age.

In the geological timescale, the Llandovery Epoch occurred at the beginning of the Silurian Period. The Llandoverian Epoch follows the massive Ordovician-Silurian extinction events, which led to a large decrease in biodiversity and an opening up of ecosystems.

The Lau event was the last of three relatively minor mass extinctions during the Silurian period. It had a major effect on the conodont fauna, but barely scathed the graptolites. It coincided with a global low point in sea level, is closely followed by an excursion in geochemical isotopes in the ensuing late Ludfordian faunal stage and a change in depositional regime.

The Sandbian is the first stage of the Upper Ordovician. It follows the Darriwilian and is succeeded by the Katian. Its lower boundary is defined as the first appearance datum of the graptolite species Nemagraptus gracilis around 458.4 million years ago. The Sandbian lasted for about 5.4 million years until the beginning of the Katian around 453 million years ago.

Stage 10 of the Cambrian is the still unnamed third and final stage of the Furongian series. It follows the Jiangshanian and precedes the Ordovician Tremadocian Stage. The proposed lower boundary is the first appearance of the trilobite Lotagnostus americanus around 489.5 million years ago, but other fossils are also being discussed. The upper boundary is defined as the appearance of the conodont Iapetognathus fluctivagus which marks the beginning of the Tremadocian and is radiometrically dated as 485.4 million years ago.

Westergaardodina is a species-rich genus of spine, U or W-shaped paraconodont known from Middle Cambrian to Lower Ordovician strata.

The Kendlbach Formation is a Late Triassic to Early Jurassic (Hettangian) geological formation in Austria and Italy. It contains the Global Boundary Stratotype Section and Point (GSSP) for the Hettangian stage at the Kuhjoch section in the Karwendel Mountains of Austria.

References

  1. ↑ National Trust for Scotland: Grey Mare's Tail Nature Reserve
  2. ↑ Hogg, James (1869). Thomson, Thomas (ed.). The works of the Ettrick shepherd. London: Blackie and Son. p.  77.
  3. ↑ Ogg, James. "GSSP for the Ordovician-Silurian Boundary". Archived from the original on 2006-06-18. Retrieved 2006-07-01.
  4. ↑ "Dob's Linn". Scottish Geology. Hunterian Museum. Archived from the original on 2011-05-18. Retrieved 2006-07-01.
  5. ↑ Underwood, C J; S F Crowley; J D Marshall; P J Brenchley (July 1997). "High-resolution carbon isotope stratigraphy of the basal Silurian Stratotype (Dob's Linn, Scotland) and its global correlation". Journal of the Geological Society. 154 (4): 709. CiteSeerX   10.1.1.502.6989 . doi:10.1144/gsjgs.154.4.0709.
  6. ↑ Lapworth, Charles (1878). "The Moffat Series". Quarterly Journal of the Geological Society. 34 (1–4): 240–346. doi:10.1144/GSL.JGS.1878.034.01-04.23.
  7. ↑ Pringle, J. (1948). British regional Geology: The South of Scotland (2 ed.). London: Her Majesty's Stationery Office (HMSO). pp.  34–46.
  8. ↑ Oldroyd, David R (1990). The Highlands Controversy. Chicago: The University of Chicago Press. pp. 217–265. ISBN   978-0-226-62635-2.

Coordinates: 55°25′47″N3°16′11″W / 55.42972°N 3.26972°W / 55.42972; -3.26972

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