Five or six miles above Mywa, the valley contracts much, and the Tambur (whose bed is elevated about 3000 feet) becomes a turbulent river, shooting along its course with immense velocity, torn into foam as it lashes the spurs of rock that flank it, and the enormous boulders with which its bed is strewn.[69] From this elevation to 9000 feet, its sinuous track extends about thirty miles, which gives the mean fall of 200 feet to the mile, quadruple of what it is for the lower part of its course. So long as its bed is below 5000 feet, a tropical vegetation prevails in the gorge, and along the terraces, consisting of tall bamboo, _Bauhinia, Acacia, Melastoma,_ etc.; but the steep mountain sides above are either bare and grassy, or cliffs with scattered shrubs and trees, and their summits are of splintered slaty gneiss, bristling with pines: those faces exposed to the south and east are invariably the driest and most grassy; while the opposite are well wooded. _Rhododendron arboreum_ becomes plentiful at 5000 to 6000 feet, forming a large tree on dry clayey slopes; it is accompanied by _Indigofera, Andromeda,_ _Spiræa,_ shrubby _Compositæ,_ and very many plants absent at similar elevations on the wet outer Dorjiling ranges. [69] In some places torrents of stone were carried down by landslips, obstructing the rivers; when in the beds of streams, they were often cemented by felspathic clay into a hard breccia of angular quartz, gneiss, and felspar nodules. In the contracted parts of the valley, the mountains often dip to the river-bed, in precipices of gneiss, under the ledges of which wild bees build pendulous nests, looking like huge bats suspended by their wings; they are two or three feet long, and as broad at the top, whence they taper downwards: the honey is much sought for, except in spring, when it is said to be poisoned by Rhododendron flowers, just as that, eaten by the soldiers in the retreat of the Ten Thousand, was by the flowers of the _R. ponticum._ Above these gorges are enormous accumulations of rocks, especially at the confluence of lateral valleys, where they rest upon little flats, like the river-terraces of Mywa, but wholly formed of angular shingle, flanked with beds of river-formed gravel: some of these boulders were thirty or forty yards across, and split as if they had fallen from a height; the path passing between the fragments.[70] At first I imagined that they had been precipitated from the mountains around; and I referred the shingle to land-shoots, which during the rains descend several thousand feet in devastating avalanches, damming up the rivers, and destroying houses, cattle, and cultivation; but though I still refer the materials of many such terraces to this cause, I consider those at the mouths of valleys to be due to ancient glacial action, especially when laden with such enormous blocks as are probably ice-transported.