Physiographic division; Siwalik and Higher Himalaya

Physiographic divisions of the Himalaya

Siwalik (Churiya) zone. The southernmost hill belt of Nepal, 10-50 km wide, altitude about 300-1500 m (up to 2000 m), lying between the MFT and MBT. Rocks are Miocene to Pleistocene molasse sediments, eroded from the rising Himalaya, mostly weak and poorly consolidated.

Lithology (three divisions)

• Lower Siwalik: alternation of fine-grained sandstone, mudstone and siltstone; red, purple and grey colours (Miocene).
• Middle Siwalik: thick, massive, coarse "salt and pepper" sandstone with some mudstone and conglomerate lenses (Mio-Pliocene).
• Upper Siwalik: poorly cemented boulder-pebble conglomerate with sandstone and clay (Plio-Pleistocene).

Structure: the strata are folded into anticlines and synclines with strike roughly parallel to the Himalaya (WNW-ESE), often steeply dipping or overturned in the north near the MBT, and cut by thrust faults such as the MBT and MFT.

Engineering problems: soft, erodible rocks, landslides, gullying, debris flow and flash floods. The sandstones are mostly too weak for good aggregate; the boulders and gravels of the Upper Siwalik conglomerate and of the river beds are the usual aggregate sources.

Higher (Great) Himalaya. Lies between the MCT and STDS; altitude about 4000-8848 m. Rocks are Precambrian to Early Palaeozoic high-grade metamorphic rocks and granites.

Lithology (three formations)

• Formation I (lower): kyanite-sillimanite schist and gneiss, with migmatite.
• Formation II (middle): calc-silicate gneiss, marble and diopside-bearing rocks.
• Formation III (upper): augen gneiss, quartzite, and the Manaslu and other leucogranite intrusions.

Structure: the Higher Himalaya is a thick slab of crystalline rocks whose foliation dips generally northward at gentle to moderate angles. It is bounded by the MCT below and the STDS above, and is folded and sheared near these boundaries. Glacial and periglacial landforms are common.

Engineering problems: very high relief, rockfall, avalanches, glacial lake outburst floods (GLOF), glacial and moraine hazards, hard rock needing blasting, and difficult access.