Crychan Forest Tracks
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Geology & landscape
Geological Period: Ordovician, Silurian
Landscape processes: Glacial
Description
Description A composite site made up of four forest track cuttings, which together form a key stratigraphic sequence in the type area for the Llandovery Series of the Silurian system. The tracks are situated in the central part of Crychan Forest, which lies a few miles north-east of Llandovery. Scrach Track shows an almost continuous section across the Ordovician -Silurian boundary, from the top of the Scrach Formation, through the Bronydd and Crychan formations. The Ashgill-Rhuddanian (Ordovician - Silurian) boundary lies within the Bronydd Formation which has graptolites of the acuminatus Biozone near its base. The locality at Cwm-coed-aeron shows informative track-side exposures in Llandovery (Silurian) rocks of the Trefawr Formation. Also exposed is the lower part of the Rhydings Formation, which has been interpreted as representing open marine-shelf conditions resulting from a late Llandovery transgression. This section yields a common brachiopod fauna and occasional graptolites, and is a key stratigraphic site in the type area for the Llandovery Series of the Silurian system. Coed Glyn Moch Track provides important sections through about three hundred metres of strata of Llandovery age (Silurian). Rocks exposed belong to the Rhydings, Wormwood and Lower Cerig formations, and contain common shelly faunas, including brachiopods. The long Tre-fawr track section exposes most of the Llandovery Trefawr Formation. The sandy mudstones of this formation contain a graptolite and shelly fauna. This section is the stratotype for the Aeronian Stage, defined at the base of the triangulatus zone on this track. The graptolite fauna of this zone is of worldwide occurrence, one reason for its selection as an indicator of the base of the stage. Near the boundary the fauna consists mainly of brachiopods, but bivalves, gastropods, cephalopods, trilobites, corals and bryozoa also occur. This is a key stratigraphic site for studies of Llandovery strata.