Most of the Netherlands a Quaternary delta, local supplies of natural stone suitable for building are rare. These include: Quaternary bog iron ores used in early medieval times (until c. 1200); erratics, deposited in boulder clay during the Saalian ice age in the northern Netherlands and gravel from the river Meuse; Miocene lithified siliceous sands, Nivelstein sandstone, used in 11th-12th century building and again in the early 20th century; and Maastrichtian deposits in the southern part of Limburg province. These provide the major supply of Dutch natural stone: A chalky limestone, mergel, used since Roman times, but in the 16th century also as far north as the central Netherlands. Maastrichtian cherts have been used very locally. Maastrichtian Kunrade limestone was also used locally and, to limited extent, outside the province of Limburg. Finally, Upper Carboniferous sandstones from the Geulle valley were used locally, mainly for farm houses. © 2006 Taylor & Francis Group.