Dob's Linn, near Moffat

Moffat

Moffat is a former burgh and spa town in Dumfries and Galloway, Scotland, lying on the River Annan, with a population of around 2,500. The most notable building in the town is the Moffat House Hotel, designed by John Adam...

, Scotland

Scotland

Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

, is the location of the Global Boundary Stratotype Section and Point

Global Boundary Stratotype Section and Point

A Global Boundary Stratotype Section and Point, abbreviated GSSP, is an internationally agreed upon stratigraphic section which serves as the reference section for a particular boundary on the geologic time scale. The effort to define GSSPs is conducted by the International Commission on...

 (GSSP) which marks the boundary between the Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 and Silurian

Silurian

The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

 periods, and marks the base of Llandovery epoch, on the geologic time scale

Geologic time scale

The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth...

. The area was first studied by Charles Lapworth

Charles Lapworth

Charles Lapworth was an English geologist.-Biography:He was born at Faringdon in Berkshire and educated as a teacher at the Culham Diocesan Training College near Abingdon, Oxfordshire. He moved to the Scottish border region, where he investigated the previously little-known fossil fauna of the area...

 in the late 19th century, whose work established graptolite

Graptolite

Graptolithina is a class in the animal phylum Hemichordata, the members of which are known as Graptolites. Graptolites are fossil colonial animals known chiefly from the Upper Cambrian through the Lower Carboniferous...

s as a method of understanding stratigraphic

Stratigraphy

Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....

 sequences. Dob's Linn was ratified as the GSSP by the International Union of Geological Sciences

International Union of Geological Sciences

The International Union of Geological Sciences is an international non-governmental organization devoted to international cooperation in the field of geology.-About:...

 in 1984.

The boundary is defined as the first appearance

First appearance datum

First appearance datum is a term used by geologists and paleontologists to designate the first appearance of a species in the geologic record. In other words, FADs represent the geologically oldest fossil of a particular species that has yet been discovered.FADs are frequently used to designate...

 of graptolite

Graptolite

Graptolithina is a class in the animal phylum Hemichordata, the members of which are known as Graptolites. Graptolites are fossil colonial animals known chiefly from the Upper Cambrian through the Lower Carboniferous...

s Parakidograptus acuminatus and Akidograptus ascensu 1.6 m above the base of the Birkhill Shale Formation. The shale

Shale

Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...

 section also contains chitinozoa

Chitinozoa

Chitinozoa are a taxon of flask-shaped, organic walled marine microfossils produced by an as yet unknown animal. Common from the Ordovician to Devonian periods , the millimetre-scale organisms are abundant in almost all types of marine sediment across the globe...

 and conodont

Conodont

Conodonts are extinct chordates resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils now called conodont elements, found in isolation. Knowledge about soft tissues remains relatively sparse to this day...

s, but neither are well preserved. Dob's Linn has been criticized for the difficulty in relating its graptolite biostratigraphic

Biostratigraphy

Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them. Usually the aim is correlation, demonstrating that a particular horizon in one geological section represents the same period...

 sequence with shallow water sequences elsewhere, although the stratotype also appears to correspond with a carbon-13

Carbon-13

Carbon-13 is a natural, stable isotope of carbon and one of the environmental isotopes. It makes up about 1.1% of all natural carbon on Earth.- Detection by mass spectrometry :...

isotope excursion in the latest Ordovician which can be identified worldwide.