What is a Stratum?

Geologists named the stratum where the caves were excavated the Jiuquan Formation. But what exactly is a stratum?

A stratum is a rock assemblage that is layered and possesses common characteristics or attributes. It typically consists of sedimentary and volcanic rocks that, with a noticeable section, are easily distinguishable from neighboring strata or layered volcanic rocks. As an example, the Gobi Formation and Jiuquan Formation situated atop the steep walls of the Mogao Grottoes are two distinct strata. Strata with identical characteristics often give rise to specific geomorphologic landscapes, which make them a crucial foundation for examining rock landscapes.

How are strata formed? The formation of sedimentary rock strata starts when mud, sand, and gravel carried by water or glaciers are deposited in low-lying rivers, lakes, or oceans. The older sediments settle at the bottom and are covered by newly-arriving sediments. Over time, the persistent pressure from the overlying layers solidifies the lower sediments, transforming them into layered rocks that eventually form a stratum.