62 km., is only 79 m., and in the next succeeding section of 2 2 km. it amounted to only 2I m., for Camp XCIII stood at an altitude of 4644 m. In a word the work of erosion is practically completed and the Bogtsang-tsangpo has entered upon a stadium in which the whole of at any rate its lower course serves merely as a through channel for the passage of the water. Hence in this part of the river the water plays in the meantime a passive role, and is no longer able to excavate its bed, that is to say make any change in its vertical depth; the energy of the stream is confined exclusively to horizontal activity, as is evident from the abandoned loops already alluded to.
The old marginal terraces are frequently pierced by the outlets of side-glens. As however the steep craggy mountain-ranges that shut in the latitudinal glen on both flanks are rather small in dimensions, most of the side-glens are also quite small. They do indeed manage to break through the terrace, but after that they hardly ever succeed in chiselling a distinguishable channel in the surface. It was only in the second half of the day's march that we passed some of these side-glens possessed of sufficient energy to reach all the way down to the river, and only one of them then contained water. In some places there were small crescentic pools along the banks, filling reaches of the river which have got cut off from it.
The valley runs at first towards the west-south-west, but afterwards inclines to the west-north-west. Sometimes it contracts, then again it widens out; but during the latter part of the day it remained on the whole steadily broader. We forded the stream at the point where it changes its direction and then for a considerable distance travelled along the slopes on the south side of the valley, that is along the top of the terrace. In the throats of the side-glens that open upon the northern bank of the river we counted in all five nomad tents. The conformation of the southern mountains now began to show up more distinctly. It was of course only
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