The rest die and fall to the bottom of a lake. Fish, crawdads and other critters eventually join them. Soils and pollens, grain by grain, wash or fall into the lake, burying the other detritus. Millimeter by millimeter, the collection builds. And years later, the saga of a lake's environment lies encrypted in the sediments beneath its waters.
History is down there, waiting to be deciphered.
Because contents and color of sedimentary layers can vary with the seasons, striations are often seen. These striations, called varves, can also be found in ancient lake sediments converted into sedimentary rock. When it's known that certain plants were introduced in an area at a particular time, the first occurrence of that plant's pollen can serve as a date marker. Natural radioactive elements can also be used to date sediment layers. For example, a decade ago, analysis of pond sediments solved an environmental puzzle, at least for some people.