Planet Earth can be conceived of as a sphere with several layered shells. An inner ‘solid core’ is surrounded by a liquid ‘outer core’ that is in turn ensconced in a relatively more viscous ‘lower mantle’. The ‘upper mantle’ is more viscous, dense than the lower mantle and this is in turn contained in a solid ‘crust’. The continental crust is thicker but less dense than the thinner but tougher oceanic crust that forms the ocean beds. The earth is about 6350 KM from the outer crust to the centre of the inner core. The crust and the outer mantle together form earth’s ‘lithosphere’. The lithosphere is in turn broken into ‘plates’ that are moving/ swimming over the less viscous inner layers of the earth.
Some 70 million years ago one such plate called the ‘Indo-Australian Plate’ was moving north at 15 cm per year when it collided with the Eurasian plate. The ocean (Tethy’s Ocean) that separated the two plates disappeared with the collision. The Indian plate went under the Tibetan plateau pushing the ocean floor under the northern plate. The lighter sedimentary rocks on the ocean bed surface, however, got folded up into layers much like the crumpling of a table cloth of the table that is squeezed under another surface. The ‘folded’ sedimentary ‘geological table-cloth’ is what we know as our magnificent Himalayan mountain range. The Indian plate continues its northward journey, though now at a more sedate 6-7 cm a year pushing up the Himalayas by 5 mm every year. Needless to say that when the Indian plate presses against the Asian plate, the pressure sends an occasional tremble that is experienced as an ‘earthquake’. The movement of plates produces a fracture or discontinuity in the earth’s crust at the boundaries of the plates that are called ‘faults’ and these are the sites for the major earthquakes.
The area of collision between the Indian and the Asian plates was over 2000 KM long and the first fold was the Tibetan Himalayas and was comprised of the thick layer of marine sediments that had got deposited on the continental shelf and slope of Indian plate. This area is rich in fossils. The next fold was of the Greater Himalayas, the high mountains that were formed out of the continental crust of the Indian plate that got metamorphosed under high temperatures and pressures resulting from the collision. The next fold was of the Lesser Himalayas that is comprised of rocks that experienced lesser metamorphism as the initial impact of the collision was over. Then followed the Himalayan foothills, the Sub-Himalayas, which were formed by the folding up of the material eroded from the higher Himalayas by the great Himalayan rivers. As the Indian plate continues to press its way under the Asian plate, the crumpled folds of the ‘geological table cloth’ are getting pulled over the plate. Thus, the Sub-Himalayan hills are getting pulled over the alluvial plains that lie to their South along a long arc called the Main/ Himalayan Frontal Thrust/Fault. By a similar process, the Lesser-Himalayas are getting thrust over the Sub-Himalayas along the Main Boundary Thrust/Fault. The Lesser Himalayas are getting subsumed under the Greater Himalayas along the Main Central Thrust.
The Sub-Himalayan foothills that border the higher Himalayas are known as the ‘Shivaliks’ (literally the tresses of Lord Shiva). The Shivalik hill range is 2400 KM long and 10 to 50 KM in width with the elevation ranging from 900-1500 metres. The Shivaliks are divided into many sub-ranges. The Shivaliks are cut by the great Himalayan rivers like the Indus, Satluj, Ganga, Yamuna and Brahmaputra. Smaller seasonal rivers like the Ghaggar meander through the hills emerging from the gaps between the sub-ranges to flow into the plains to the south. The Lesser Himalayas rise sharply to the north and while these are abutting the Shivaliks in some areas yet at other places the two parallel ranges are separated by structural valleys called ‘Duns’. The Shivaliks are composed chiefly of sandstone and conglomerate rock formations that have been created by the solidification of rock material and gravel eroded by the rivers from higher Himalayas.
The Shivaliks have numerous choes and seasonal nadis that have deposited silt, sand and coarse gravel in a belt surrounding the foothills. This 5-8 KM wide belt is well drained and is called the ‘Kandi’ area in Punjab (Bhabar in UP). The coarse kandi area is highly permeable and the water from the springs and choes percolates deep so that groundwater levels are in the range of 300-400 feet. The area has historically been thickly forested due to replenishment of soil by the silt carried by the choes during monsoons and has been popular for game hunting. The Shivaliks have a sharp slope to the south. The higher permeability and high temperatures in summers allow only a dry-scrub forest on the southern slopes. The northern slopes are gentler. The Kandi belt then makes the transition to the great alluvial Indo-Gangetic plain. The Shivaliks are a repository of vertebrate fossils including those of several extinct species including the giant tortoise and the horned-giraffe.
Morni hills of Haryana form a part of the Shivalik range.
Geologists study the structure of earth’s crust by classifying layers of rock into distinctive ‘formations’. A rock formation is a layer of rock that is distinct from the surrounding layers and is also extensive enough and thick enough to be plotted on a map by the geologist. Rock formations are generally named after the geographic name of the location where the rocks are well exposed. They may also be named after the dominant rock type comprising the formation.
The Sub-Himalayas in the area of Morni are classified by geologists into two main rock formations- the Shivalik Group and the the Sirmur Group.
The Sirmur Group is older and was formed some 40 to 20 million years ago. It includes the Dagshai, Subathu and Kasauli formations. The Subathu formation is the oldest and consists of thick layers of gritty, grey-brown clay with irregular bands of impure limestone. The Subathus give way to the more striking purple and green sandstone and red clay of the Dagshai formation. The Dagshai formation merges into the upper Kasauli formation that has a softer, coarser and greener sandstone and shale with absence of red clay.
The Shivalik Group is more recent and was formed in the period 15 million years to about 800,000 years ago. Rocks were formed out of the eroded rock material that was deposited into the lake like basin created by the Indian plate (peninsular India) striking against the Tibetan plateau. There were differences in the type of material deposited over the geological ages leading to the Shivalik Group rocks being divided into 3 sub-groups: a) The Lower Shivalik Group (also known as the Nahan Formation). The sedimentary rocks in this formation are characterized by the grey sandstone (formed by compacting of medium grained sand < 2mm), siltstone (formed by compacting of fine grained silt <.063 mm) and red mudstone (formed by compacting of extremely fine grained clay <.004 mm. The sandstone is hard and resistant to erosion. b) The Middle Shivalik Group includes the Nagri formation and the Dhok Pathan formation. Nagri formation includes sandstone of coarser material and grey-black mudstone.The Dhok Pathan formation includes fine to medium grained bright grey sandstone, bright grey/ brown siltstone and red/orange/yellow clay. The Dhok Pathan formation is evidenced in the sandstone cliffs to the north of the plains at Dhanoura village. c) The Upper Shivalik Group consists of the Saketi, Pinjor and the Boulder Conglomerate Formations. Broadly, the group consists of conglomerates of pebbles (4-64 mm), cobbles (64 -256 mm) and boulders (> 256 mm). The loose, disorganized boulder-sized conglomerate occuring with buff-coloured sandstone and mudstone is seen on the hill road from Pinjore to Morni.
During the period of the Lower Shivalik deposition (the material that formed the rocks of the Nahan formation), finely eroded material was brought by the peninsular rivers that flowed into the basin created by the disappearance of the Tethys sea. At the time of the Middle Shivalik deposition, the Himalayas had risen and the Himalayan rivers flowed south-east with greater force, eroding and carrying coarser materials. During the Upper Shivalik sedimentation, the Himalayan uplift was complete exposing rocks to rapid erosion by strong streams and rivers that carried pebbles and boulders with it. The monsoon is believed to have increased in intensity increasing the water flow from lower to upper Shivalik period and to have thereafter receded.
- Geology and Mineral Resources of Haryana, Geological Survey of India (Northern Region), 2012
- Himalayan Frontal Fold-Thrust Belt, NW India: Geometry, Structural Evolution, and Hydrocarbon Prospects; Dilip K. Mukhopadhyay and Premanand Mishra (2007)
- Memoirs of the Geological Survey of India (Vol III); Thomas Oldham (1865)