What is the erosion rate of mountains?
This may not answer your question, but it indicates that erosion is extreme at high altitudes.


  More than 60 percent of the sediment delivered to the world's oceans in the prehuman world originated from erosion in mountainous areas with elevations greater than 3 km above sea level.
  Muddy Waters -
  More sediment is entering rivers, but less makes its way to the sea
  - Sid Perkins


Only about 1% of Earth's area is above 3 km.


  
  (Fig. 2.4 of Deep-Sea Biology: A Natural History of Organisms at the Deep-Sea Floor) (via DavePhD)