Kathmandu

Tank leads us on a geological field trip on the way to Kathmandu, including a stop at the Main Central Thrust, just ouside of Kodari (upper right corner of the map), then cross into the rocks of the Lesser Himalaya (brown and lavender on the map), which are exposed in a great embayment in the Main Central Thrust. We stop in several locations to examine these lower grade metamorphic rocks. As we near our destination, we cross the Main Central Thrust one more time, and onto the young rocks of the Kathmandu "piggyback" Basin (yellow on the map).

The adjective, "piggy-back," derives from the fact that sediment accumulates on top of a moving thrust fault, in this case the south-directed Main Central Thrust, and it carried along with it. The largest such basins in the Himalayas are the Peshawar, Jalalabad, and Kashmir Basins of Pakistan and northwestern India. The Kashmir Basin began to form approximately at the time of the earliest movement on the Main Boundary Thrust, and contains over 1 kilometer of sediment.

Kathmandu is located in a broad valley where the preexisting drainage system was disrupted and a temporary lake formed when the front of the thrust was uplifted during movement. Sediment accumulated in the resulting basin until the river cut through the thrust toe and the original drainage pattern was reestablished. The maximum thickness of sediment exposed in the Kathmandu Basin is 280 meters (920'). The lowest sediments are Pliocene in age and consist of alluvial fans and debris flows shed off the uplifted area to the south. A sequence of sediments carried by rivers, consisting of interbedded sandstones, siltstones, and mudstones with thin selvages of low-grade coal was laid down over these deposits. The southern highland was breached at this point, and the through-going drainage reestablished. However, there is a similar sequence of upper Pleistocene sediments, indicating a second period of damming. The present Bagmati River and its tributaries are through-going, eventually joining the Ganges drainage system, but it is possible that in the future another period of basin sedimentation will occur.

A local myth preserves an almost Jungian memory of this geological history. The legend, recounted in a Buddhist text dating from 1558, tells how a sacred lake inhabited by Nagas and other deities once filled the valley. From this lake sprouted a golden lotus plant, which was studded with precious stones. The lotus was recognized as a manifestation of the Primordial Buddha, and Manjushri, the Bodhisattva of Wisdom, came to the lake to venerate it. He decided that his followers should settle in this holy area, and using his sword, cut through the confining hills draining the lake. As the water drained, the lotus flower was transformed into Swayambhu Hill, which is sacred to both Buddhists and Hindus and the site of a large stupa. In a similar Hindu rendering, Pradyumna, the son of Krishna, strikes the blow to empty the lake and free the goddess of the Bagmati River, Vagvati, from a demon.

The next morning is devoted to sight-seeing. We visit the Buddhist stupa at Bodhnath (above) and the Hindu temple to Shiva at Pashupurtinath (left) before heading to the airport for our departure.