The Ludlow Group are geologic formations deposited during the Ludlow epoch of the Silurian period in the British Isles, in areas of England, Ireland, Scotland, and Wales. Formations This group contains the following formations in descending order: Tilestones Formation, Downton Castle Sandstone formation (90 ft./27.7 m), Ledbury Formation shales 270 ft./83 m), Upper Ludlow sub-group formation (140 ft./43 m), Aymestry Limestone Formation (up to 40 ft./12.3 m), Lower Ludlow sub-group formation (350 to 780 ft./108 m-240 m). Geology The Ludlow group is essentially shaly in character, except towards the top, where the beds become more sandy and pass gradually into the Old Red Sandstone. The Aymestry limestone, which is irregular in thickness, is sometimes absent, and where the underlying Wenlock limestones are absent the shales of the Ludlow group graduate, downwards into the Wenlock shales. In Wales the group is typically developed between namesake Ludlow and Aymestrey, and it occurs also in the detached Silurian areas between Dudley and the mouth of the Severn. In the Lake District the Silurian Coldwell beds, forming the upper part of the Coniston Flags, are the equivalents of the Lower Ludlow. They are succeeded by the Coniston Grits (4,000 ft./1,230 m), the Bannisdale Slates (5200 ft./1,600 m) and the Kirkby Moor Flags (2,000 ft./615 m). In the Silurian areas of southern Scotland, the Ludlow rocks are represented in the Kirkcudbright Shore and Riccarton districts by the Raeberry Castle Beds and Balmae Grits (500–750 ft.). In the northern belt within Lanarkshire and the Pentland Hills, the Lower Ludlow portion consists of mudstones, flaggy shales, and greywackes; and the upper Downton Castle Sandstone part is made up principally of thick red and yellow sandstones and conglomerates with green mudstones. The Ludlow Group formations of Ireland include the Salrock beds of County Galway, and the Croagmarhin beds of Dingle Peninsula of County Kerry. Lower Ludlow sub-group The Lower Ludlow rocks are mainly grey, greenish and brown mudstones and sandy and calcareous shales. They contain an abundance of fossils. The Lower Ludlow series has been zoned by means of Monograptus species of Graptolites by E. M. R. Wood. The zonal forms in order from older/lower to younger/upper are: Monograptus vulgaris; Monograptus nilssoni; Monograptus scanicus; Monograptus tumescens; and Monograptus leintwardinensis. In Denbighshire and Merionethshire the upper portion of the Denbighshire Grits belongs to this horizon. Those from lower to upper are: the Nantglyn Flags; the Upper Grit beds; the