Until joined by its right-hand affluent the Aldan,which deflects it to the north-west; then, after receiving its most important left-hand tributary, the Vilyui, it makes its way nearly due N.
The Altai, the still unnamed complex of the Minusinsk Mountains, the intricate mountain-chains of Sayan, with those of the Olekma, Vitim and Aldan all arranged en echelon - the former from N.W.
The orography of the Aldan region is little known; but travellers who journey from the Aldan (tributary of the Lena) to the Amur or to the Sea of Okhotsk have to cross the same plateau and its border-range.
Basaltic lavas are reported to have been found in the Aldan region.
It is this Wale ' circumstance that facilitated the rapid invasion of Siberia Wal er l by the Russian Cossacks and hunters; they followed the omm courses of the twin rivers in their advance towards the east, and discovered short portages which permitted them to transfer their boats from the system of the Ob to that of the Yenisei, and from the latter to that of the Lena, a tributary of which - the Aldan - brought them close to the Sea of Okhotsk.
In 1643 Poyarkov's boats descended the Amur, returning to Yakutsk by the Sea of Okhotsk and the Aldan, and in1649-1650Khabarov occupied the banks of the Amur.
This conformation is seen in the valley of the Us in West Sayan, in that of the upper Oka and Irkut in East Sayan, in the valley of the Barguzin, the upper Tsipa, the Muya and the Chara, at the foot of the Vitim plateau, as also, probably, in the Aldan.