The forces of weather on the surface of the Earth can change the minerals that make up rocks. When mechanical weathering processes destroy old rocks, softer silicate minerals such as olivine and plagioclase may dissolve away, leaving behind harder minerals, such as quartz. In chemical weathering, the atoms react with oxygen or water in the environment. (Think of iron exposed for several months to the water and air outdoors. The iron rusts, and rusting is a kind of chemical weathering.) In the presence of water, some minerals transform into more hydrous minerals. Other minerals, such as calcite (calcium carbonate), dissolve completely in water over time. Some silicate minerals, such as quartz and garnet, are more resistant to weathering than other silicates, such as feldspar and mica.

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.