Most fossils form when plants or animal remains are quickly covered in SEDIMENT (sand or mud), like the stuff that collects at the bottom of rivers, swamps or the ocean.
(plants or animal remains; are quickly covered; in SEDIMENT) (the stuff; collects; L:at the bottom of rivers, swamps or the ocean)
plants or animal remains the stuff
are quickly covered collects
in SEDIMENT
L:at the bottom of rivers, swamps or the ocean