Location of the Caledonian/Acadian mountain chains in the Early Devonian Epoch. Present day coastlines are shown for reference. Red lines are sutures, capitalized names are the different continents/super-terranes that joined during the Caledonian orogeny.
Avalonia started as a chain of volcanoes on the northern margin of Gondwana. It became a drifting microcontinent. The Iapetus Ocean in front of it slowly shrank.
Avalonia collided with other continents. It ended up in the middle of Pangaea. When Pangaea broke up, Avalonia's remains were divided by the rift which became the Atlantic Ocean.