It is clear that plate tectonics were at work, from the existence of old mountains (such as the Urals in the U.S.S.R. and the Appalachians in the U.S.) that were formed more than 200 million years ago.
(the Urals in the U.S.S.R. and the Appalachians in the U.S.; were formed; T:more than 200 million years ago)
the Urals in the U.S.S.R. and the Appalachians in the U.S.
were formed
T:more than 200 million years ago