(Neuwied district). Furthermore, the Lahn-Dill-Kreis, the Mayen-Koblenz district, the Rhein-Lahn-Kreis and the Limburg-Weilburg district each have shares of the Westerwald. If Sieg is taken as the Westerwald's northernmost limit, then the Rhein-Sieg district likewise belongs here, at least in parts (for example the Siebengebirge and the communities of Eitorf and Windeck). Transport connections The Westerwald and its outer edges are crossed by stretches of 8, 42, 49, 54, 62, 255, 256, 277, 413 and 414, over which there are connections to the Autobahnen A 3 (Cologne–Frankfurt), A 45 (Dortmund–Aschaffenburg) and A 48. The most notable railway is the Cologne-Frankfurt high-speed rail line with minor stops at Montabaur and Limburg an der Lahn. The High Westerwald is served by Siegerland Airport for private use and special touristic charters, south of Burbach. Geology Geologically, the Westerwald is part of the Rhenish Massif, and likewise represents a heavily eroded remnant of a great Variscan mountain system which in the Mesozoic characterized a great deal of Europe. The Devonian bedrock is covered by volcanic masses from the Tertiary, particularly basalt and tuffs. Economically important, besides slate, limestone and clay quarrying, were, and still are, iron and its processing industry between Rheintal (Unkel, Linz) and the lower Wied, pumice gravel in the Neuwied Basin, various mineral springs and, once, brown coal mining. The whole Westerwald region lay under a tropically warm arm of the sea in the Palaeozoic (600 to 270 million years ago). This sea deposited layers of sediments many kilometres thick into the Variscan geosyncline, which were heavily folded in the orogeny that followed. The towns of Siegen and Koblenz on the Westerwald's north and southwest edges even gave two Lower Devonian layers, with their colourful slates, their names. The upper mountain layers are formed of volcanic strata made of basalt containing tuffs. In a few areas, slate and clay have long been quarried, the latter notably in the so-called Kannenbäckerland, but also in a few other places where the clay is worked into the salt-glazed grey Westerwald Pottery with cobalt blue decoration. The pottery industry is centred on Höhr-Grenzhausen. Exports, particularly to Italy, are also important (more than one million metric tons each year). In the mid 16th century, potters from Raeren in Belgium migrated into the Westerwald, bringing with them some of their moulds. This type of pottery was taken to the New World and was found in the early Chesapeake