Imagine a grain of quartz sand lying on the bank of the upper Mississippi River, at the boundary between Minnesota and Wisconsin. The grain is picked up by rising springtime water and travels hundreds of kilometers downstream, through a series of locks and dams, past the relict boomtown of Cairo, Illinois, where the Mississippi swells with input from the Ohio River. The river grows several more times through the lower Mississippi River valley as tributaries contribute water and sediment to its sinuous flow. Along the way, the grain may be stored underground, perhaps for a long time, or exposed to daylight as it rests temporarily on river bars or banks.
Ultimately, the quartz grain reaches the Mississippi delta of southeastern Louisiana, where the river meets the ocean and deposits its sand and mud. The grain is surrounded by others as it settles to the bottom of a shallow bay and...
