Tibet Geography(2)
  
 

For reasons explained in the detailed description of the expedition (see below), we were not able to float the entire distance of the Mekong in Tibet . We aborted the expedition a few miles below the village of Xilong , about 80 miles below Changdu, and hiked to the west up a nearby side canyon to Chala Shan, a village where there was road access to the main highway between Zogong and Changdu. On this hike we passed from Precambrian schist at river level (10,000') into granitic and granodioritic intrusives (11,500 - 13,000') with glaciated Paleozoic carbonate roof pendants exposed on high peaks (up to 21,000').

Between Xilong and Yangjing, our only view of the Mekong was at the Zoca Bridge , about two-thirds of the way between Changdu and Yangjing. The canyon widens here, and the Tibetans farm Quaternary gravel terraces 200' to 300' above the river. These terraces are remanants from the last glacial retreat (about 10,000 years ago) when rivers were choked with sediment. They are common throughout the river canyons of Tibet . Rather than scouring deeper canyons, rivers deposited glacial gravels, raising the elevation of their beds as much as several hundred feet. This also raised the base level for side canyon deposits, causing the development of terraces that could be grazed and farmed when humans migrated into the canyons about two thousand years ago.

Photo by Travis Winn

At Yangjing (located on a large, high terrace in far southeastern Tibet ), the river cuts through Cenozoic volcanics not shown on the geological map. In association with these volanics, there are low sulfur content hot springs that are utilized by a local resort and warm springs that are exploited for their high salt content.

Photo by Lui Li

One of the objectives of the expedition was to field check Chinese geologic maps of the area that were produced from aerial photos, but the difficulties encountered on the river resulted in a failure to achieve this object. In other areas, Chinese geologic maps have been fairly accurate given their scale (1:1,000,000), but floating the Mekong in Tibet is so challenging that further geologic work should be done by hiking into and along the canyon where possible.