Here's why:

* Sedimentary Rock Formation: These rocks form from layers of sediment (like sand, mud, or shells) that settle at the bottom of bodies of water or on land. Over time, the pressure from the layers above compacts and cements the sediment together, forming rock.

* Principle of Superposition: This fundamental principle in geology states that in undisturbed sedimentary rock layers, the oldest layers are at the bottom, and the youngest layers are at the top.

* Fossil Formation: When organisms die, their remains can be buried under these sediment layers. Over millions of years, the pressure and minerals in the sediment can preserve these remains, forming fossils.

Therefore, the deeper you go into a sedimentary rock formation, the older the fossils you're likely to find.