This is consistent with a rate of upward expansion of kilometres per million years, which in turn implies that the formation is geologically young and tectonically exceptionally active; the substance of the terrain is accordingly crumbly.
Firstly, it is the prime source of sediments for the plain of Terai — torrents flowing down the steep gradients erode material from the fragile rock and transport it downstream.
The streams slow down as they flow onto the plain, and in consequence they deposit their burden of silt, thereby filling or clogging the streambeds and raising their levels.
On the credit side however, the silt replenishes the nutrient content of the soil of the downstream lowland plains, which produce more than 80% of Nepal's agricultural food supply.
In spite of its protective cover of vegetation, the Siwalik is subject to severe erosion that creates gullies and promotes landslides.