Here's why:

* Weight and Pressure: The weight of overlying rock layers creates enormous pressure on the rocks below. This pressure compresses the rocks, forcing them closer together and increasing their density.

* Lithostatic Pressure: The pressure exerted by the weight of overlying rocks is called lithostatic pressure. It increases with depth, meaning the deeper you go, the greater the pressure.

* Compaction and Cementation: The pressure also promotes compaction, where pore spaces between grains are squeezed out, and cementation, where minerals precipitate in those spaces, further increasing density.

However, there are some exceptions to this general rule:

* Rock Type: Some rock types, like sandstone, may have a more porous structure than others like granite, even at similar depths. This can affect density.

* Mineral Composition: The density of rocks also depends on the specific minerals they are composed of.

* Deformation: Even in undeformed strata, localized areas of folding or faulting can create variations in density.

Overall, while density increases with depth in undeformed strata, it's important to consider the specific factors that can influence density in individual cases.