Complex sentence: Geologists divide the iron-bearing rocks of the eastern Mesabi Iron Range into several layers: its four main divisions from the top down are named the Upper Slaty, Upper Cherty, Lower Slaty and Lower Cherty.
Split into simpler sentences: Geologists divide the iron-bearing rocks of the eastern Mesabi Iron Range into several layers. The four main divisions from the top down are the Upper Slaty, Upper Cherty, Lower Slaty and Lower Cherty.