Category: wales

Neale Monks (UK) The rocks we know in Britain and Ireland as the Carboniferous Limestone were laid down between 363 and 325 million years ago, during a period when global sea levels were particularly high, a condition that geologists refer to as a transgression. The climate was tropical, and the warm, shallow seas that covered much of the British Isles teemed with life. Consequently, the Carboniferous Limestone is often highly fossiliferous, and good exposures can yield vast numbers of crinoids, brachiopods, corals, bryozoans and other types of marine fossil. Despite being known as the Carboniferous Limestone, one thing notably absent from this formation is coal. Coal is made from the fossilised remains of trees, and the forests and freshwater swamps where those trees grew could only develop once sea level had dropped. Coal-bearing sediments weren’t laid down until the second half of the Carboniferous Period, when sea level was relatively low. International stratigraphyThe International Commission on Stratigraphy refers to the interval of time between 359 and 299 million years ago as the Carboniferous Period, but, historically American geologists recognised two periods instead: the Pennsylvanian and the Mississippian. These were roughly equivalent to what geologists elsewhere considered the Lower and Upper Carboniferous, so the ICS has standardised the Pennsylvanian and Mississippian as the two epochs within the Carboniferous. However, it isn’t quite as simple as sea level dropping in the middle of the Carboniferous and all the subsequent sediments of the period being terrestrial in nature. What tended to happen was … Read More

Paul D Taylor (UK) Ask a geologist to name a fossil bryozoan found in the rocks of the British Isles and the most likely answer will be Fenestella. The net-like fossils of Fenestella are especially abundant in the Carboniferous Limestone (Figs 1 and 2), although the genus, as used in its broadest sense, is also present in the Silurian, Devonian and Permian deposits of Britain. Fig. 1. Colony of Fenestella (s.l.) nodulosa from the Lower Carboniferous of Calcot Quarry, Halkyn Mountain, Flintshire. Branches forming the characteristic meshwork fan outwards from the colony origin. Fig. 2. Large colony of Fenestella (s.l.) flabellata from the Carboniferous Limestone of Fife in Scotland. Fracturing of the meshwork is evident. While Fenestella dominates almost all bryozoan assemblages found in the British Carboniferous, a variety of other bryozoans are commonly found. Some Carboniferous bryozoans inhabited reefs or mounds, others were components of non-reef marine communities where they lived together with brachiopods, crinoids and corals at a time when the British Isles was situated close to the equator. All Carboniferous bryozoans constructed immobile colonies consisting of numerous individual zooids, with crowns of tentacles used to capture tiny planktonic algae floating in the water around. Our knowledge of the diversity of Carboniferous bryozoans in the British Isles has increased enormously during the last 50 years through the studies of David E Owen, Ron Tavener-Smith, Adrian J Bancroft and Patrick N Wyse Jackson. Yet, and in common with bryozoans from other geological periods, Carboniferous bryozoans are too often perceived … Read More