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

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The Cambrian is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma to the beginning of the Ordovician Period 486.85 Ma.

<span class="mw-page-title-main">Ordovician</span> Second period of the Paleozoic Era

The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era, and the second of twelve periods of the Phanerozoic Eon. The Ordovician spans 41.6 million years from the end of the Cambrian Period 486.85 Ma to the start of the Silurian Period 443.1 Ma.

<span class="mw-page-title-main">Silurian</span> Third period of the Paleozoic Era

The Silurian is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.1 million years ago (Mya), to the beginning of the Devonian Period, 419.62 Mya. The Silurian is the third and shortest period of the Paleozoic Era, and the third of twelve periods of the Phanerozoic Eon. 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.

<span class="mw-page-title-main">Charles Lapworth</span> English geologist (1842–1920)

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

<span class="mw-page-title-main">Graptolite</span> Subclass of Pterobranchs in the phylum Hemichordata

Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding organisms are known chiefly from fossils found from the Middle Cambrian through the Lower Carboniferous (Mississippian). A possible early graptolite, Chaunograptus, is known from the Middle Cambrian. Recent analyses have favored the idea that the living pterobranch Rhabdopleura represents an extant graptolite which diverged from the rest of the group in the Cambrian.

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The Hirnantian is the final internationally recognized stage of the Ordovician Period of the Paleozoic Era. It was of short duration, lasting about 2.1 million years, from 445.2 to 443.1 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.

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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 Hirnantian Age and precedes the Aeronian Age.

<span class="mw-page-title-main">Llandovery Epoch</span> First Series of the Silurian

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<span class="mw-page-title-main">Katian</span> Second stage of the Upper Ordovician

The Katian is the second stage of the Upper Ordovician. It is preceded by the Sandbian and succeeded by the Hirnantian Stage. The Katian began 452.8 million years ago and lasted for about 7.6 million years until the beginning of the Hirnantian 445.2 million years ago. During the Katian the climate cooled which started the Late Ordovician glaciation.

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 458.2 million years ago. The Sandbian lasted for about 5.4 million years until the beginning of the Katian around 452.8 million years ago.

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<span class="mw-page-title-main">Highlands controversy of Northwest Scotland</span> 19th-century geological controversy

The Highlands controversy was a scientific controversy which started between British geologists in the middle of the nineteenth century concerning the nature of the rock strata in the Northwest Highlands of Scotland. The disagreement stemmed from the apparent ages of the strata, particularly the, now confirmed, existence of older rock above younger rock as well as duplicated and inverted strata, which could not be satisfactorily explained by contemporary geology. This rock formation and surrounding controversy were the impetus for Albert Heim's theory of thrust faulting, which, in conjunction with anticlines and imbrication, are now commonly accepted as the primary geological mechanisms that created the Northwest Highlands rock strata.

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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 18 June 2006. Retrieved 1 July 2006.
  4. ↑ "Dob's Linn". Scottish Geology. Hunterian Museum. Archived from the original on 18 May 2011. Retrieved 1 July 2006.
  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.

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


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