Conglomerate is one of many sedimentary layer stones. It is notable for not only supporting aquifers but also frequently containing large clusters of puddingstone which will also support the aquifer.
In Real Life
Conglomerate is a type of clastic sedimentary stone composed of large, well rounded grains, at least 2 millimeters in diameter; if the grains are angular, it is called a breccia (which is not present in Dwarf Fortress). Conglomerate which is poorly sorted, that is, with grains of various sizes rather than mostly the same size, is called puddingstone.
Boulder of conglomerate with cobble-sized clasts.