one of the things mentioned that I never knew was that the mountains here once used to be much, much bigger, possibly as tall as the Himalayas.

This is quite surprising to be honest. Mountains are constantly eroded, that is true but the size of erosion is surprising. The truth is, landscapes are reshaped all the time by the power of water, kilometers of dry land is swallowed by seas where the beach is not protected. Also, new dry land appears where lakes disappear due to drought. These are interesting things and I think if you dig deeper into the history of any place, you can find surprising phases.