High mountain

I found this article in regards to the Sierra Mountains growing faster then expected.  I have been their a couple of times in life and its magnificent.  The mountains always seemed so majestic.  I've always was curious to find out why how mountains get so big.

Geologists studying deposits of volcanic glass in the western United States have found that the central Sierra Nevada largely attained its present elevation 12 million years ago, roughly 8 or 9 million years earlier than commonly thought.  The finding has implications not only for understanding the geologic history of the mountain range but for modeling ancient global climates. (yahoo.com)

The higher the mountain, the more pronounced the rain shadow effect is and the greater the decrease in the number of heavy hydrogen isotopes in the water that makes it across the mountains and falls on the leeward side of the range. By determining the ratio of heavier to lighter hydrogen isotopes preserved in volcanic glass and comparing it with today's topography and rainwater, researchers can estimate the elevation of the mountains at the time the ancient water crossed them.

My question how do these scientists get all this information.  What exactly do they study in order to get this information.  How do they determine how old these elevations are.  I would like to get inside the head of the scientist.