Dislodged by the action of water and wind, surface rock and mineral materials are transported and deposited at new sites. These sediments often change chemically in the water to allow for a variety of new minerals to develop. Some materials in the original rock, such as gold, may become concentrated enough during transportation to form a placer deposit.

Audio Transcript
The last three cases deal with minerals that are formed in a sedimentary environment. This is an environment in which rock making up mountain ranges, for example, or plains or plateaus, undergoes the process of physical and chemical deterioration, or weathering. The activity of water and wind often transport these products to new sites where they are deposited. During the process of transport, these sediment materials often become concentrated in the bottom of a streambed or on the flood plain of a stream bed, especially if the minerals are quite heavy. 

In the one case, you see shallow pans containing flecks of gold, rubies, and sapphires, and almandine garnets, and peridot that have been concentrated by the activity of running water. Such mechanical concentration of mineral materials is a placer deposit. Gold and rubies and sapphires and chromite and almandine garnet are the kinds of minerals that typically are formed under the placer conditions. Shown, too, is the kind of rock that contains these minerals before they are weathered free. 

To the right in the last series of two cases are displayed minerals that are formed in the environment of sedimentary deposition, some even perhaps through the activity of bacteria. The vast deposits of iron oxide, or hematite, in the north central United States were formed under sedimentary conditions. And so, too, is bauxite, the only major source of aluminum formed under these conditions.