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). If the grains are rounded pebbles, the conglomerate is called puddingstone.