Conglomerate sedimentary rock is made of pebbles surrounded by a matrix of sand or mud. Sedimentary rocks reveal the most about past history of an area. These sediments after being deposited in places like a delta, beach, or dune, then harden into a rock. Thus clastic rocks contain information about their parent rocks, what transported the sediments, and the environment they finally deposit in. Important features of clastic sedimentary rocks include grain size, sorting, rounding, color, and structure.
Grain size is a clue to the energy of the transporting agent, ranging from coarse to fine.
A fast-moving river can transport coarser particles than calm pond water so larger sediments are found nearer rushing rivers than near lagoons. From coarse to fine, sediments range in size from gravel to sand, silt, and clay. These harden into rocks named conglomerate, sandstone, siltstone, and shale. Being in rushing water is like being tossed in a washing machine. As the grains knock against each other and the rocks at the sides of the stream, their rough edges smooth.