PALAEOZOIC/Silurian 189 preserved: the trilobites and corals are the best known The bioherms consist mostly of stromatoporoids and calcareous algae, with subsidiary tabulate corals Above the Wenlock Limestone on Wenlock Edge the strata pass up without substantial break into the overlying Elton Formation, which is of Ludlow age and extends laterally into the type Ludlow area, some 25 km away The Ludlow Series The mediaeval town of Ludlow lies south-west of Much Wenlock in Shropshire, England, and the low cliffs to the south and west of the town form a shallowly plunging anticline, which includes the type rocks of the Ludlow Series and its two stages, the lower Gorstian and the upper Ludfordian The formal base of the Ludlow Series and Gorstian Stage is located in Pitch Coppice quarry, 4.5 km southwest of Ludlow, where the Much Wenlock Limestone Formation is succeeded conformably by the Elton Formation This boundary correlates with the base of the Neodiversograptus nilssoni graptolite Biozone The 110–225 m thick Elton Formation consists largely of blocky fine siltstones, which were deposited in the middle and deeper parts of the shelf and contain many macrofossils, particularly brachiopods, trilobites, and cephalopods, and a large variety of microfossils, particularly conodonts, acritarchs, chitinozoa, and ostracods Above the Elton Formation is the 62–105 m thick Bringewood Formation, which includes the Aymestrey Limestone of Murchison, famous for its banks of the large pentameride brachiopod Kirkidium knightii The formal base of the Ludfordian Stage is at Sunnyhill Quarry, 2.5 km south-west of Ludlow, where the Bringewood Formation is overlain by the Leintwardine Formation The base of this unit is defined in beds correlating with the base of the Saetograptus leintwardinensis graptolite Biozone The 35 m thick Leintwardine Formation and the overlying 55 m thick Whitcliffe Formation both consist of siltstones and mudstones with, again, common brachiopods and cephalopods and rich microfaunas and microfloras At the top of the Whitcliffe Formation is the Ludlow Bone Bed (see above), which is overlain by the Downton Castle Sandstone The latter can be correlated indirectly with beds of the Pridoli Series in its type section in the Czech Republic The Pridoli Series The Pridoli (technically spelt Prˇ ı´dolı´, but the Czech accents are often omitted) Series was of much shorter duration than the other three series of the Silurian and as a result is not subdivided into stages Its type section is in the disused Pozˇ a´ ry Quarry at Rˇ eporyje, in the western suburbs of Prague, Czech Republic There the Pozˇ a´ ry Formation is a uniform facies of dark platy limestones with intercalations of calcareous shale Brachiopods occur commonly throughout, with abundant cephalopods at certain horizons and rich microfossil floras and faunas The base of the Pridoli Series is defined at the first appearance of the graptolite Monograptus parultimus, a key constituent of the Monograptus ultimus graptolite Biozone This horizon is close to the gradational boundary between the underlying Kopanina Formation and the overlying Pozˇ a´ ry Formation The Pridoli Series lies entirely within one conodont biozone, that of Ozarkodina eosteinhornensis, whose first and last appearances fall below and above the Pridoli rocks The upper limit of the series is about m above the base of the Lochkov Formation, which overlies the Pozˇ a´ ry Formation The top of the Pridoli Series is defined by the internationally agreed base of the Devonian System and its basal Lochkovian Stage, which is taken at the base of the Monograptus uniformis graptolite Biozone, whose ‘golden spike’ is at Klonk, also in the Czech Republic and about 23 km south-west of Pozˇ a´ ry Other Key Silurian Areas New York Niagara Falls, in New York State, USA, and bordering Ontario, Canada, runs over a Silurian escarpment of relatively flat-lying rocks There the Medina Group, consisting of the Whirlpool Sandstone, the Power Glen Shale, the Grimsby Formation, and the Thorold Sandstone, is of Llandovery age and is overlain paraconformably by the Clinton Group, which is of Wenlock age The latter unit includes the very fossiliferous Rochester Shale, famous for its trilobites and other invertebrate fossils, and other formations (Figure 4) Above this lie the Lockport Group, of late Wenlock and Ludlow age, and the Salina Group, which includes many anhydrites of latest Ludlow and Pridoli age Much of the sequence is relatively unfossiliferous, since the limestones have been largely replaced by dolomites Norway and Sweden The graben running north–south through the Oslo region of Norway includes a rich succession of Silurian rocks The Solvik, Ryttera˚ ker, and Vik formations form a Llandovery sequence that conformably overlies the latest Ordovician rocks and underlies the Wenlock rocks The latter include the Skinnerbukta, Malmøya, and Steinsfjorden formations The Solvik