Trefawr Track, a forestry road north of Cwm-coed-Aeron Farm, Llandovery

Llandovery

Llandovery is a market town in Carmarthenshire, Wales, lying on the River Tywi and the A40 road.The town is served by Llandovery railway station, where there is a park and ride to Llanelli and Shrewsbury via the Heart of Wales Line...

, Wales

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

, 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 Rhuddanian and Aeronian stages of the 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...

 period 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 GSSP was ratified 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 the 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...

 Monograptus austerus sequens (the base of the Monograptus triangulatus biozone). The section is primarily mudstone

Mudstone

Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...

, which yields an abundance of shelly fauna

Fauna

Fauna or faunæ is all of the animal life of any particular region or time. The corresponding term for plants is flora.Zoologists and paleontologists use fauna to refer to a typical collection of animals found in a specific time or place, e.g. the "Sonoran Desert fauna" or the "Burgess shale fauna"...

s. The graptolite Diplograptus elongatus occurs immediately below the boundary.