* Fossils form in sedimentary rock: Fossils are the preserved remains or traces of ancient life. They are most commonly found in sedimentary rocks because these rocks are formed from the accumulation of sediments like sand, silt, clay, and organic matter.

* How sedimentary rocks form: Over time, these sediments are compacted and cemented together, creating sedimentary rock. The process of fossilization often occurs during this formation.

* Other rock types: Igneous rocks (formed from cooled magma or lava) and metamorphic rocks (formed by the transformation of existing rocks under heat and pressure) are unlikely to contain fossils.

Therefore, if a layer contains clam fossils, it is almost certainly a sedimentary rock.