Which sedimentary rock consists mainly of rounded gravel-sized clasts?

Conglomerate is a sedimentary rock consisting mainly of rounded gravel-sized clasts.

The rounded fragments usually began as pebbles or cobbles transported by rivers, beaches, or glaciers. Repeated movement causes collisions and abrasion that smooth their edges. Burial then compacts the sediment and cements it with minerals such as silica, calcite, or iron oxides.

A conglomerate can contain many rock types, depending on its source area and transport history. Its clasts are generally larger than 2 millimetres, while the material between them is called the matrix. The rock’s composition can therefore provide clues about erosion and ancient environments.

Conglomerate is often confused with breccia. Both are coarse clastic sedimentary rocks, but conglomerate has mostly rounded clasts, whereas breccia has mostly angular clasts. The difference records how much transport and abrasion the fragments experienced.