Complex sentence: Deposition resumed some 35 Ma in the Oligocene epoch on a flood plain that developed in the area; sluggish streams migrated laterally over the surface, laying down cobbles, sand, and mud.
Split into simpler sentences: Deposition resumed some 35 Ma in the Oligocene epoch on a flood plain that developed in the area. Sluggish streams migrated laterally over the surface, laying down cobbles, sand, and mud.