Here's why:

* Fossil Formation: Fossils form when organisms die and are buried under sediment (like sand, mud, or volcanic ash). Over time, the sediment compacts and hardens into rock, preserving the remains.

* Types of Rock: Mammal fossils are most commonly found in sedimentary rocks, which are formed by the accumulation and cementation of sediments. These rocks include sandstone, limestone, and shale.

* Examples: Famous mammal fossil discoveries, like the "Lucy" Australopithecus skeleton or the "Sue" Tyrannosaurus Rex skeleton, were all found in sedimentary rock formations.

So, if you're searching for mammal fossils, keep your eyes peeled for rocky outcrops and sedimentary layers!