Graptolites occur primarily in limestones in the Ordovician succession in the Marathon region. Study of thin sections of these limestones reveals that most of them were lime muds; however, oolitic and bioclastic limestones also bear graptolites. Integration of the thin section study of the several rock types occurring in the Ordovician sequence with analysis of the associated sedimentary features enables recognition of four major environments. During the Early Ordovician mud was accumulating in the region. Local sectors of it were frequently exposed for brief periods; also, for a short time, small patch reefs flourished. In the early part of the Middle Ordovician strong currents swept the area and calcium carbonate oolites and aggregates formed. Aligned biserial graptolites and the direction of dip of the cross-laminae indicate that the last current to pass over the region came from the southwest. Then a tidal mud flat developed during the latter part of the Middle Ordovician. It was occasionally swept by currents bearing pelagic organisms. Aligned biserial graptolites again indicate that the direction from which the current came was southwest. Finally during latest Middle Ordovician and all of the Late Ordovician the region was predominantly accumulating lime mud under reducing conditions.