Conglomerate is a clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts). The clasts in conglomerate rocks can be made up of a variety of materials, including rock fragments such as pebbles, cobbles, and boulders of different types of rocks, such as granite, limestone, sandstone, shale, or even volcanic rocks like basalt. Mineral fragments may also be present. The spaces between the clasts are typically filled by a mixture composed of varying amounts of silt, sand, and clay, known as matrix). Conglomerates often begin when a sediment consisting mainly of pebble- and cobble-size clasts is being deposited. The finer-size sand and clay, which fill the spaces between the larger clasts, is often deposited later on top of the large clasts and then sifts down between them to fill the interstitial spaces. After compaction, the precipitation of a chemical cement between the grains will bind the sediment into a rock.

In summary, conglomerate is a type of sedimentary rock that is composed primarily of rounded or water-worn pebbles, cobbles, and boulders, which are known as clasts, and is typically cemented together by a matrix of finer-grained sedimentary material, such as sand, silt, or clay.