Some sedimentary rocks are made up of large pebbles or even boulders that are held together by natural cement made with finer particles. Scientists call the rocks conglomerates if they contain rounded pebbles and breccias if they are made up of angular grains. The roundness of the pebbles inside conglomerates indicates that flowing water had smoothed out the fragments before they were embedded in the rock. Well-sorted conglomerates, with grains all about the same size, come from long-term water flows, whereas short, rapid flows lead to poorly sorted conglomerates. The roughness of the fragments within breccias indicates that they may have not traveled far from their origins. When Apollo astronauts brought Moon rocks back to Earth from 1969 to 1972, many of their specimens turned out to be breccias, formed from the fragmented debris of meteorite impacts on the lunar surface. Unlike Earth, the Moon does not have flowing water and blowing winds to smooth the edges of rock shards, so the components of lunar breccias are extremely jagged.

Gravel, pebbles, and boulders all go into making the coarsest-grained sedimentary rocks, known as conglomerates (if the rock pieces are rounded, as if by water) or breccias (if the grains are angular). Geologists, using magnifying tools if necessary, can identify the kind of rocks or minerals that the grains are made of. Thus, researchers can tell whether one type of rock or many types of rocks went into making the conglomerate or breccia and whether or not these fragments resemble nearby or distant rocks. The overall orientation of the grains within the rock may help scientists determine the direction of the ancient river flow, glacier movement, or rock slide that led to the deposit of the grains and pebbles.