In sedimentary geology, maturity describes the composition and texture of grains in clastic rocks, most typically sandstones, resulting from different amounts of sediment transportation.
There are two components to describe maturity, texture and composition.
Texture describes how rounded and sorted the sample is while composition describes how much the composition trends toward stable minerals and components (often quartz).
One measure of this maturity is the ZTR index which is a measure of the common resistant minerals found in ultra-weathered sediments: zircon, tourmaline, and rutile.
A sediment sample from the lower (downstream) portions of a stream is likely to be more mature than one found upstream, since the original sediment has been subject to more abrasion as it travels downstream.