Travels in the Tibet (English Edition)

We rode a long distance on the old lake bottom and perfectly level
stretches of clayey mud. Afterwards the soil was of fine gravel, and as
hard as though it had been compressed by the weight of a steam-roller.
Only in an isolated drainage basin can such level expanses occur among
huge mountains. Weathering, precipitation, flowing water, storm and wind
work together in levelling the land.