Conglomerate is a terrigenous clastic sedimentary rock, which means the rock itself is made up of broken pieces of other rocks, cemented together in a matrix of fine grained materials. The particles found in conglomerate are large, ranging from 2mm to boulder sized. Some of the larger formations of conglomerate have had pieces measuring around the size of buses.

The particles in conglomerate are rounded, lacking sharp or angular edges, which are found in the sedimentary rock breccia. The roundness comes from the method in which the particles were deposited. Conglomerate is usually formed in highly active stream beds which are subject to violent flooding. The continuous rushing water rounds the edges of the pieces, which are then cemented together.

Though the most common source rock for conglomerate is granite, the particles can come from any rock that is mechanically weathered. Because a sedimentary rock can be recycled, and become another sedimentary rock, it is possible to have what is called a conglomerate of conglomerate, which is conglomerate that has been broken in to pieces, and then cemented together with other pieces of conglomerate, forming a new sedimentary rock.