Geology Reference
In-Depth Information
Map of the Tsangpo River, Tibet, showing the Nyang River, the town of Bayi (where our hotel was located), and the mo-
raine dam at head of Tsangpo Gorge. The reconstructed extent of the lower paleo-lake is shown in black.
Looking at the map, there was no obvious dam to hold back our newly discovered
ancient lakes. What kept them from draining down into the Tsangpo Gorge? Many miles
downstream, right at the head of the gorge, we discovered glacial debris plastered on both
sides of the valley confirming that a massive tongue of ice had once plunged down the
25,000-foot-high peak of Namche Barwa and blocked the river. The two levels of terraces
extending far upstream indicated that a wall of ice and mud dammed the river, not just once
but time and again, backing up a great lake that filled the valley.
As you might imagine, ice doesn't make a very good dam. Once the lake filled enough
to float or breach the dam, a rush of liberated water roared down the gorge, scouring out
everything in its path. Upstream of the gorge, we found horizontal stripes of silt plastered
onto the valley walls. Here were old shorelines confirming that the ice advanced to block
the river over and over again, most likely during cold glacial periods or at times when
Search WWH ::




Custom Search