Sedimentary layer

From DwarfFortressWiki

Sedimentary stone layers contain hematite, limonite, magnetite, tetrahedrite, bituminous coal, and lignite.

Sandstone, siltstone, mudstone, shale, claystone, rock salt, limestone, conglomerate, dolomite, flint, chert, and chalk layers are sedimentary.

The name of a sedimentary layer appears in a lighter shade on the embark menu.

Sedimentary layers are, on average, the most economically valuable of the five mineral layers. They are your only source of bituminous coal and lignite, vital for fueling non-magma forges and for the coke steel-making requires (especially if you lack trees). Even if magma is present, only sedimentary layers contain bauxite, the only rock and mechanism (other than Hidden Fun Stuff) which can withstand the heat of magma. They are much richer in iron ores than other rock formations. Several sedimentary rocks - dolomite, chalk, and limestone - are fluxes, required to make steel and also of double value when turned into trade goods or furniture.