Actually, mountain building can't be explained AT ALL on the RE.
"Not long ago in a geological sense, the flat plain from New Jersey to Florida was under the sea.
At that time the ocean surf broke directly on the Old Appalachian Mountains. Previously the
southeastern part of the mountain structure had sunk below the sea and become covered with a
layer of sand and mud, thickening seaward. The wedgelike mass of marine sediments was then
uplifted and cut into by rivers, giving the Atlantic coastal plain of the United States. Why was it
uplifted? To the westward are the Appalachians. The geologist tells us of the stressful times
when a belt of rocks extending from Alabama to Newfoundland was jammed, thrust together, to
make this mountain system. Why? How was it done? In former times the sea flooded the region
of the great plains from Mexico to Alaska, and then withdrew. Why this change?"
The birth of the Cordilleras—"again the mystery of mountain-making clamors for solution."
And so on all over the world. The Himalayas were under the sea. Now Eurasia is three miles or
more above the bottom of the Pacific. Why?
The problem of mountain-making is a vexing one: many of them [mountains] are composed of
tangentially compressed and over-thrust rocks that indicate scores of miles of circumferential
shortening in the Earth's crust. Radial shrinkage is woefully inadequate to cause the observed
amount of horizontal compression.
Even authors of textbooks confess their ignorance. "Why have sea floors of remote periods
become the lofty highlands of today? What generates the enormous forces that bend, break, and
mash the rocks in mountain zones? These questions still await satisfactory answers."
(from Worlds in Collision)
Here is book dedicated entirely to the mountain building mysteries of the earth:
http://www.truthseekersministries.org/files/Velikovsky-Earth-in-Upheaval.pdf