Simplify the following text for a Grade 10 native English-speaking person.
Original: The Highland Boundary Fault was active during the Caledonian Orogeny , a plate tectonic collision which took place from Mid Ordovician to Mid Devonian periods ( 520 to 400 million years ago ) , during the closure of the Iapetus Ocean .
Simplified: The Highland Boundary Fault was active during the Caledonian Orogeny . This was a plate tectonic collision which took place from Mid Ordovician to Mid Devonian periods ( 520 to 400 million years ago ) , during the closure of the Iapetus Ocean .