Mountain formation takes 10s of millions of years. Compared to a ~4.6 billion year old Earth, I suppose that's relatively quick.
The fact that even on very tall supposedly billions of year old mountains are found sea shells from recent geological periods should cause the open minded to realize that large movement have occurred in the past.
The tectonic process which raised the mountains is obviously younger (more recent) than the rocks and fossils that make up the mountains, although molten rock is often injected into preexisting rocks during the mountain building process.
An interesting example is the Matterhorn where very old (billion years or so old) rock sits *atop* younger rock, the explanation being that the older rock layers were folded over the younger ones in the tectonic 'collision' which raised the Alps (the ongoing collision between the African Plate and European Plate).
“the explanation being that the older....”
Or perhaps, “The theory being....”
Or “The general consensus being....”
Or “The prevailing winds of thought being.....”