Geological layout of Darjeeling
The hilly tract of Darjeeling represents a wonderful uniqueness in its geo-environmental layout. Major part of its landform is composed of eroded rocks caused by southerly flowing streams and an entire cross section of different tectonic units. Throughout the hilly tract, there is approximately uniform lithology, structure, climate, soil and vegetative covers. Experts believe that the units formed by tectonic movements are found to be in the reverse order of stratigraphic superimposition, and is represented by Siwalik and Gondwana systems. However, the inner Himalayas, features the thrusted sheets of Daling and Darjeeling group of crystalline rocks. The contact between different groups of rocks in this region is represented by thrusts, curving at high angles towards north.
The thorough study of various formations of the Darjeeling Himalaya, will help you better understood the geographical layout of the region.
Raised Terraces: This is a fringe formed recently along the hills, especially at the confluences of the rivers. Such fringy terraces are composed of gravels, pebbles and boulders mixed with sand and clay. It is semi-consolidated, stratified with gradual upheaval at places. The area with intensified high-level terraces is also called the Terai. In the Teesta valley at Kalijhora, a 40 m high terrace is found.
Siwalik: Formed of mudstones, sandstones, shale and conglomerates, along with the bands of shale and lignite, Siwalik system is found in the Hill Cart Road and along the Teesta River.
Damuda Series: Coarse-grained hard sandstone, quartzites, carbonaceous shale and slates, Damuda series are found, immediately after the Siwalik system. This series are physically similar to the Gondwanas of Indian peninsular region. With the maximum width of about 2.5 km, Damuda us situated along the Teesta valley. The maximum thickness is about 1000m and it stretches from ENE to WSW.
Daling Series: Comprising of chlorite shales, phyllites and schist associated with quartzite, Daling series rests over Damuda series. Found along the Teesta River and the stretches of the Tindharia –Paglajhora on the Hill Cart Road, quartz and feldspar are the most prominent features of this series. With increasing metamorphism upwards, slates form the lowest bed of this series.
Darjeeling Gneiss: The Dalings gradually grade into the more metamorphosed rocks, forming Darjeeling Gneiss. With irregular rocks of varying degrees between 40 to 70, Darjeeling gneisses are highly foliated. Two prominent sets of joints form Darjeeling gneiss, one running roughly NW-SE and the other NNW-SSE. However, the general direction of the hill spurs accordingly with the joint directions.
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