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). Conglomerates which are poorly sorted, that is, the grains of various sizes rather than mostly the same size, are called Puddingstone.
Boulder of conglomerate with cobble-sized clasts.