The sediment, made up of sand, mud and pebbles constitute sedimentary rocks, which develop from other rocks that have been worn down by the environment over the years.

In order to determine the age of the layers of rocks that have accumulated over the millions of years on our planet, we need to look at sedimentary rocks, which are accumulations of rock sediments.

There are three basic types of sedimentary rocks:

Clastic sedimentary rocks

such as breccia, conglomerate,sandstone, siltstone, and shale are formed from mechanical weathering debris.

Chemical sedimentary rocks

such as rock salt, iron ore,chert, flint, some dolomites, and some limestones, form when dissolved minerals precipitate from solution.

Organic sedimentary rocks

such as coal, some dolomites, and some limestones, form from the accumulation of plant or animal debris.
More specifically, there are a number of processes that are involved in the formation of sedimentary rock:

Weathering (aka erosion)

which is the result of the friction of waves, transportation where the sediment is carried off by a current and deposition and compaction, where the sediment is flattened.

Lithification (aka rock formation)

develops as the sediment pressure squeezes the sediment into layered solids.
By reviewing the different layers of sediment, scientist can tell when rock formations and minerals began to materialize.