Complex sentence: The most productive zone is called the "18 inch layer" consists of a series of laminated or varved lime muds containing abundant fish and other fossils and are easily split along the layers to reveal the fossils.
Split into simpler sentences: The most productive zone--called the "18 inch layer"--consists of a series of laminated or varved lime muds containing abundant fish and other fossils. These are easily split along the layers to reveal the fossils.