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.

Scientists say that clastic sediments are well-sorted when the individual grains are all about the same size. When big chunks and tiny grains are mixed together, the sediments are considered poorly sorted. In general, the distribution of size of the particles in a clastic sedimentary rock may indicate how fast the water or wind transported the grains before they were deposited and ultimately became cemented into rock. The faster the flow, the larger the particles it can push. In some cases, unusually dense particles, such as nuggets of gold or magnetite, drop to the bottom of a stream or river sooner than similarly sized grains of lighter minerals. Also, after a lengthy period of movement through turbulent water, less resistant particles wear down or disintegrate altogether, leaving the more durable mineral grains behind. All these clues help geologists figure out the stories behind these fascinating rocks.