Sedimentology is the study of sediments (sand, mud and the like) and the sedimentary rocks that form from them by various processes, including burial in the Earth’s crust. By studying the shapes of the layers in the rocks, and the composition of the rocks, a geologist can usually tell how the original sediment formed. For example, think of ripples in the sand on a beach. These get buried and preserved under more sand and eventually become rock. By looking at the preserved ripples, the shapes of the layers in which they occur, and the context (the closely associated different sorts of rock) it is possible to recognise an ancient beach in what might now be a cliff eroded into an ancient sandstone. The same is true of a coral reef, or a soil, or the ash from a volcano, and many others.
Some sediments are especially distinctive. Those formed around hot springs in a “hydrothermal system” are an example. Here we see chemical compounds (minerals) precipitated out of the hot water as it cooled. This kind of sediment is quite different from, for example, those formed in the ocean, with its beaches, reefs and so on.
A geologist looking at sedimentary rocks sees ancient rivers, lakes, the oceans, a volcanic eruption, a desert sand dune, a former tropical jungle: everything we see about us now, and more.