Geology is a science, related to the scientific study of the earth. This scientific discipline, geology is relatively young because it has been around for just over 200 years. In geology we study everything that is found in the earth, geological materials and the process they go through called geological process.
Some of the geological materials are:
- Rocks, soil
- Lava, magma
- Water, ice
- Petroleum products, coal, natural gas
- Iron, gold, diamond (ores)
Some of the geological process examples are:
- Mountain building
- River flooding, drought
- Volcanic eruptions
- Ore deposits
- Oil and natural gas formations
- Erosion, Avalanches and landslides
- Glacier melting, water evaporation and precipitation
- GLOF (Glacier Lake Outburst Flood)
- Groundwater flow and hot springs
The science of earth can broadly be divided into physical geology and historical geology.
Hagen (1969) was among the earliest people to propose the physiographic subdivision of Nepal.
- Terai – Northern part of the Indo-Gangetic basin made up of Recent Alluvium spreading about 10-50 km in width.
- Churia Range (Siwaliks) or Sub-Himalaya and Dun Valleys – The Sub-Himalaya (Foreland basin) is made up of Siwalik or Churia Group of sedimentary rocks. The sediments from this subdivision cover the Dun Valleys and is also 10-50 km in width.
- Mahabharat range – is a 20-30 km width Lesser Himalayan unit made up of sedimentary, metamorphic and igneous rocks
- Midlands – is a 40-60 km wide Lesser Himalayan unit made up of sedimentary, metamorphic and igneous rocks with thrust sheets
- Fore Himalaya – Lesser and Higher Himalayan units spreading across 10-50 km width.
- Higher Himalaya and Inner Himalayan Valleys – Higher Himalayan unit is made up of high grade metamorphic rocks and also spreads across 10-50 km in width.
- Tibetan Marginal Range – The range consists of Tibetan or Tethys Himalayan unit made up of fossiliferous sedimentary rocks spreading across 10- 60 km width.
Siwaliks is indicated by the yellow line in the following map (map from Chauhan, P.R.):
- The Siwaliks are made up of mudstones, siltstones, sandstones, and conglomerates. They were deposited in a variety of fluvial environments including piedmonts, outwash plains, channels, floodplains, and oxbow lakes.
- They comprise a fining-upwards sequence on the scale of individual cycles but a coarsening-upwards succession as a whole.
- The deposits are frequently divided into the Lower, Middle, and Upper Siwaliks ranging in age from Middle Miocene to Early Pleistocene. Such lithological subdivisions are diachronous and the equivalent units become successively younger from the northwest towards the southeast.
- The thickness of a given lithostratigraphic unit of the Siwaliks may change significantly in its lateral direction (i.e., within a few tens of kilometres). It is noticed mainly in the Middle and Upper Siwaliks (e.g., in the Arung Khola area, Kamala River basin, and the Sahajpur–Godavari area).
- The Upper Siwaliks exhibit disconformable contacts in a number of places. In a few locations, they are transgressive over the Middle and Lower Siwaliks indicating past prodigious tectonic movements.
- The Siwaliks are very incompetent (soft) and have buckled into noncylindrical plunging folds with curved hinges in map view (i.e., concave or convex towards the foreland) implying some amount of shortening in the direction tangential to the fold belt.
- There are areas with overturned Siwalik strata. Such beds are sometimes found just at the boundary with the Terai plain.
- The Siwalik beds are frequently truncated by north-dipping as well as south-dipping thrust faults. Like the folds, the thrusts too are convex or concave towards the foreland. There are also some tear faults disrupting these thrusts in the direction essentially perpendicular to their strike.
- Some intermontane valleys are located within the Siwalik belt.
- There are a few Pre-Siwalik outliers in central and east Nepal.
- Contrary to a general belief, there is no single and continuous Himalayan Frontal Fault or Main Frontal Thrust outcropping throughout the Nepal Himalaya and beyond; in a number of locations the Siwalik folds abruptly disappear in the Terai plain