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The Appalachian Mountains began forming approximately 1.5 billion years ago. About the same time that sea animals were first evolving bones. The carbon that became the coal under them was deposited approximately 300 million years ago when they formed the central continental divide of the Pangea supercontinent. That was when they were at their highest, estimated to have been about the same height as the modern Alps.
Same vein, the Canadian/Laurentian Shield has areas dating back as far as 4.2 billion years, recall a geo prof in uni suggesting it would have been extremely tall, Wikipedia suggests 12km.
Stuff gets unreal to me at geological timescales.