If you look at the moutains in the background (on the right of the picture in particular), it would appear that we see erosion of volcanic ash deposit into a valey, then with rains/snow melt, further depositing into the lower wet weather river bed and washed away. Given the state of the deposits, I would questimate approximately 300,000 to one million years of erosion.
Are the Himalayas volcanic in origin?
Not far from here as the crow flies, at 14200 ft near a village called Langzha, is a big plain of fossils which indicate that a few million years ago it was part of the sea bed.
If I start a thread with a link to a page on another site (an Indian motoring and travel site), will it be allowed or will I be zotted? On the same thread, a travelogue of what is called the "Spiti Loop", you can see some of the fossils. The bigger and better ones have already been collected by travellers and the inhabitants of the villages.