What are the two periods of fold mountain building in the last 400 million years?

Fold mountains are significant geographical features formed through tectonic forces that bend and compress the Earth’s crust. The two primary periods of fold mountain building within the last 400 million years are known as the Caledonian and the Armorican or Variscan orogenies.

The Caledonian Orogeny occurred during the late Silurian to early Devonian periods, around 490 million to 395 million years ago. It primarily formed the mountain ranges in Scotland and Scandinavia as a result of the collision between the North American and Eurasian tectonic plates.

The Armorican Orogeny, succeeding the Caledonian, took place during the late Paleozoic era and occurred around 400 million to 300 million years ago. This was primarily caused by the collision of the African and Eurasian tectonic plates, contributing to mountain building in regions such as western Europe.

These two periods represent significant geological events that shaped the landscape of Europe and other areas, leading to the formation of notable mountain ranges. Thus, identifying these periods as Caledonian and Armorican reflects their importance in understanding the processes of mountain building and plate tectonics in Earth's history.