Final answer:
An aquifer layer is most likely made of cracked rocks, such as limestone or sandstone, which can act as underground reservoirs.
Step-by-step explanation:
An aquifer layer is most likely made of cracked rocks. Cracked rocks, such as limestone or sandstone, can form porous layers that allow water to pass through. These layers can act as underground reservoirs, storing and providing water for wells or springs.
Learn more about Composition of aquifer layers