It closed about 500 m.y. ago and gradually folded the eastern margin of proto-North America to create the Appalachian Mts. over the following 100 m.y. or so.
(It; closed; T:about 500 m.y. ago) (It; gradually folded; the eastern margin of proto-North America)
It It
closed gradually folded
T:about 500 m.y. ago
the eastern margin of proto-North America