Short essay on The Peninsular Block

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This morphologically polygenetic and com­plex, relatively stable, landmass extends from the southern margin of the Great Plains up to the coastal margins of the country and covers an area of about 1.6 million km2. Its northern boundary may be taken as an irregular line running from Kachchh along the western flank of the Aravalli Range to near Delhi, and thence roughly parallel to the Yamuna and the Ganga as far as the Rajmahal Hills and the Ganga Delta’ (Spate, 1967, p. 14). Recent studies have shown that it provides the basement platform for the thick deposits of the Ganga alluviums and also of the Himalayan rocks.

This Peninsular massif is the core around and upon which different acts of geological drama were staged and all have left their imprints in some form or the other. ‘This is a solid crust-block or shield which has remained a stable land-mass of gread rigidity, and has been unaffected by any fold­ing movement generated within the earth during the later geological periods. The only structural distur­bances to which these parts have been susceptible are of the nature of vertical, downward or upward, movements of large segments within it, between vertical (radial) fissures or faults’ (Wadia, 1975, P-3).

This Peninsular massif, a part of the super continent of ‘Gondwanaland’ till its rupture and drifting sometimes in the mid-Mesozoic era (about 200 million years ago), lay somewhere near the South Pole along with Australia, South Africa, South America and Antarctica. It is formed essentially by a great complex of rocks ranging from the Achaean to the Aryan groups. According to R.P. Singh the Peninsular India has passed through following land­scape cycles which have their bearing on the geomorphology of this region (Singh, 1967).

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1. Pre-Dharma Landscape-the geomorphic history of the Peninsular India begins with the pri­meval original solid crustal surface which was ex­posed to the forces of denudation and sedimentation for a long period of time. These sediments (known as pre-Dharwarian sedimentary) were buckled, folded and metamorphosed several times yielding place to basal rocks of granites and gneisses. These rocks witnessed intrusions of magma whose traces are preserved in the form of charnockites of Nigeria, Palin and Shivery. It is surmised that five ancient Geosynclines might have existed before the Cambrian period: (a) Dharma geosynclines (extending from present Dharma to Mysore), (b) Eastern Ghats I geosynclines, (c) Sapura geosynclines, (d) Ravalli m geosynclines, and (e) Delhi geosynclines.

2. Dharma Landscape Cycle-the Dharma landscape cycle begins with the formation of initial mountains out of the aforesaid geosynclines. Prior to the pre-Cambrian period these mountains were re- diced to pen plain stage although according to Heron Ravalli were peneplained during Mesozoic era. The activities of erosion, sedimentation, magmatic intrusions and lava flows continued for longer per- rood of time. Numerous longitudina consequent and I some transverse streams were formed with the Achaean folding. Evidences of sedimentation (Dhanjori Conglomerates) and igneous activity I (Dallam and Dhanjori lava flows) have been founding these valleys. During Dharma there were three main I transgression seas-Cuddapah, Vindhyan and Bijawar. Amongst these Cuddapah was extending® from Nigeria to Andhra Pradesh and some parts of Madhya Pradesh, Vindhyan from Ravalli to Sasaratl I and Bijawar covering parts of the Son and the Narmada valleys.

3. Cuddapah-Vindhyan Landscape Cycle-This M cycle starts with the upliftment of the Cuddapah and me the Vindhyan formations. Rivers descending from H the Western Ghats and southern slopes of the Sapura I deposited their sediments into the Cuddapah Sea while that originating from the Ravalli and the northern slopes of the Sapura’s accumulated thief detritus into the Vindhyan Sea. Thus there were I three axes of the Archaean drainage-Western Ghats drainage, Sapura drainage and Ravalli drainage. I the upliftment of the Vindhyan ranges led to the origin of various consequent, subsequent am I consequent streams and readjustment was noticed in H the existing Ravalli and Sapura drainage systems.

4. Vindhyan Glaciations-The Vindhyan de-1 posits exhibit traces of glaciations, e.g. boulder bed me the Son valley and tallies of the Ken valley. While I the upper parts of the Vindhyan ranges were covered I with glaciers freezing the sources of the streams and producing per glacial conditions in the surroundiniH areas the truncated rivers were active in their middle I and lower reaches. After the removal of the glacier the gneissic pen plain surface was covered by ma H
rime deposits due to the transgression of the sea during Pleistocene period and the drainage was deranged. During this period there was upliftment in Gujarat area, emergence of land in Cambay region and formation of raised beaches, sand dunes, la­goons and alluvial deposits in the coastal zone.

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5. Cambrian Landscape Cycle-by the end of the Cambrian period the obliteration of the Vindhyan glaciations yielded place to new landscape cycle which led to substantial changes in the relief and drainage of the area. Ravalli suffered intensive denudation but could attain peneplanation with the beginning of the Cretaceous period. Although there was significant reduction in relief of the Peninsular India due to Cambrian cycle of erosion, but still recognizable relief could be preserved.

6. Carboniferous Landscape Cycle-During the Carboniferous period entire Godwin land (in­cluding South America, Africa, Australia, India, Antarctica and Malagasy) was extensively glaciated. The axis of these glaciations was over the Ravalli from where the ice sheets spread in eastern and south-eastern directions covering a vast area from Rajmahal to Godavari and from Ranging to Nagpur. There were fluctuations in Carboniferous sea-levels due to advancements and retreat of ice sheets which resulted into transgression of sea on land. The previ­ous cycle of erosion was terminated because of the spread of ice sheets over the existing topography.

The lower courses of a few streams became trun­cated because these could not be covered by the advancing ice sheets. Táchira basin was in immunological stage as a result of which boulders were deposited up to the depth of 15 to 30 m. Later immunological stage ended with the onset of warm conditions which facilitated iron rich rocks in the Ranging and Pinched ranges. The Hercynian move­ment caused rupture of the peninsular gneissic sur­face and formed several tectonic troughs in the basins of the Mahanadi, the Deodar and the Godavari rivers wherein sinking valleys allowed sedimenta­tion up to thousands of meters in thickness due to the melting ice sheets.

7. Godwin Landscape Cycle-the Carbon­iferous glaciated surface is separated from the sub­sequent Godwin sedimentary by an unconformity. Godwin sediments are still preserved in tectonic basins although in much modified form. The Rajmahal basalt was also formed due to outpouring of lava during this period. Since much of the Peninsula was peneplained up to the end of the Mesozoic era (early Cretaceous period) the areas not subjected to inten­sive denudation were characterized by the gradational landforms. The general slope of the Peninsula during this time was from south to north but later on this slope axis changed due to tilting. The Nigeria and Cardamom hills still present evidence for south-north gradient. Although most of the Godwin erosion surfaces have been obliterated due to subse­quent erosion but their traces are visible in Tamil Nadu, Maharashtra and Madhya Pradesh. Accord­ing to R.P. Singh the relics of this surface may also be traced in the Ravalli and Vindhyan region.

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8. Fragmentation of Gondwanaland and Post- Godwin Landscape Cycle-The fragmentation of the Gondwanaland took place in the mid-Mesozoic era as a result of which Peninsular India drifted towards the north and joined the Asian land mass. The marginal areas of the Peninsula were many times transgressed by the sea leading to horizontal structure of lomenta formation in Tiruchchirappalli and Baghlemra region. New landforms were added over the Godwin surface during post-Godwin cycle whose relicts are seen in the form of beveled surfaces of Oozy and Cardamom hills, high summits of the Eastern Ghats, highlands of the Vindhyan and Ravalli ranges and relics in Maharashtra and Madhya Pradesh.

9. Cretaceous-Eocene Lava Flow-Two lava flows one in Rajmahal area during Cretaceous pe­riod covering an area of 3,97,000 km2 and second in Maharashtra area (from Kathiawar to Nagpur and from Malawi to Dharma) during Eocene period covering an area of 5,18,000 km2 buried the earlier landforms and topography. The prolonged denuda­tion of basaltic surface by fluvial process resulted into the ‘development of hills, ridges, valleys and plains etc. The scarps of basaltic plateau are still undergoing the process of parallel retreat due to back wasting.

10. Cenozoic Landscape Cycle-the Peninsu­lar India was peneplained before Tertiary epoch. During Cenozoic cycle the Western Ghats were formed due to the subsidence of a large area, between Africa and the west coast of India (giving birth to the Arabian Sea). Western Ghats determine the drain­
age axis of the present drainage system except the Narmada and Tapi which How in rift valleys, formed due to the tensions generated by the Himalayan progeny. Some of the rivers were rejuvenated due to Tertiary upheaval, tilting and warping which is evi­denced by the presence of multi-level plateaus, high lands, nick points, gorges and waterfalls. The raised beaches denote negative change in sea level.

Traces of Cenozoic landscape cycle are found in Karnataka plateau, Nilgiri, Cardamom-Annamalai hills and are characterized by relict denuded hills and monadnocks. Extensive warping occurred dur­ing Quarternary period which rejuvenated most of the rivers and yielded place to nick points, waterfalls and gorges in their longitudinal profiles. During this time changes were also noticed in the drainage pattern. Due to the transgression of sea in Cambay- Ahmadabad region marine deposits were laid down over the peneplained gneissic surface, lava surface and older rocks.

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Erosion Surfaces

The almost plain topographic surfaces hav­ing undulating ground surface and remnant low reliefs caused by dynamic wheels of denudation processes and cutting across geological formations and structures are generally called erosion explanation surfaces’ (Singh, 1998, p. 309). ‘Hence, pen plains, pan plains, pediplains and planes of marine, erosion are all erosion surfaces in the accepted sense of the term.’ (Small, R.J., 1970).

The face of the Peninsular India has been carved out of its architectural plans of the past in relation to the grinding action of the dynamic wheels of denudational cycles. Its morphology, even in its bare outline, suggests a polygenetic and complex phenomenon (Singh, R.P., 1962). Singh (1956) on the basis of denudation chronology and other factors have identified a number of erosion surfaces in the highlands of Chotanagpur and has projected them in the Peninsular India along with other surfaces of the Peninsular Plate (Singh, 1969). Among other schol­ars who have made regional study of erosion sur­faces mention may be made of R.S. Dubey (1971) of Rewa Plateau, Y.J. Parthasaradhi and R Vaidyanadhan (1974) of Nilgiri, Savindra Singh and Renu Srivastava (1976) of the Belan Basin, Savindra Singh (1978) of Ranchi Plateau, D.P. Satpathi (1981) of Singhbhum, Savindra Singh and D P. Upadhyay (1985) of S Chotanagpur and A. Kumar (1985) of Chotanagpui region.

R.P. Singh (1987) has identified four maid erosion surfaces in the Peninsular India. Besides there are various erosional surfaces with incomplel cycle in different parts of the region.

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Nilgiri plateau surface of over 2500 m. is the core of surface history in the Peninsular India, as outlines polygenetic and polycyclic surface. Great segment of Nilgiri plateau is an undulating surfed having general elevation of 2134 m rising to ova 2500 m in Dodabetta environ (Anaimudi 2695 m Dodabatta 2637 m).

A 1500-2600 m. surface may be identified when we consider a few selected peaks and landsitj their vicinity. Some significant peaks viz. Vavi Mala(2339m), Biligiri Regan Hills(1750m),MouK Abu (1722 m.), Kasugai (1646 m), Mahendragi (1501) etc. are outstanding examples. Shillong Pla-j teau describes surface of 1829 m. It descends and also indicates surfaces of over 1200 ten, 670 m and 305 m.

A 1000-1500 m. surface appears when wi i ntegrate peaks 1 ike Mahabalesh war (1438 m.), Satpud Hills (1350 m), Nokrek (1412 m), Main Pat (115! m), Dhupgarh (1350 m), Parasnath (1366 m), West­ern Ghats (average height 1200m), Paehmarhi(l 11 m), Girnar (1117 m), Maikal Hills (1036 m Amarkantak (1036 m) and Meghalaya (Shillon] Plateau). Some peaks remain outstanding in Hit landscape throughout the Peninsular Plate for the) either define surfaces, relict landscape or compara ble summit levels. Pats with over 1000 meter spreading in Netarhat, Jamira Pat, and Amarkantak etc; constitute topographic surface.

As we descend from Ootacamund to Mysore the granites and gneissic country of Karnataka, Andhra Pradesh, Bellary, Madhya Pradesh and Bihar ap­pears as 600 m surface with break of slope to outline 300 m surface which wraps the 600 m surface almost on every side. Ranchi peneplained surface is an appro­priate term to apply for 600 m surface throughout the Peninsular Plateau. Lower Ranchi peneplained spreads below the upper Ranchi peneplained surface which ii a lower erosion surface of 300 m are largely intruded by plutonic intrusions. In the Archaean rocks gneiss is very common which, more or less possesses a foliated or banded structure. It is composed of varying amount of white or pink orthoclase, a little oligoclase, bluish quartz in small grains, biotite, muscovite, hornblende and acces­sory minerals like tourmaline, epidotic, chlorite, gar­net, spinel, graphite, magnetite, limonite etc.

Another popular rock of the Archaean group is the schist. It is a comparatively fine-textured rock not much different in composition from gneiss ex­cept for a slight decrease in the quantity of ortho­clase. It is very much foliated and may be biotite-, graphite-, sillimanite-, chlorite-, epidote-, garnet schist depending upon the preponderance of one or the other of its accessory constituents. Other Achaean rocks include granites (Karnataka, Rajasthan, Tamil Nadu), augite-granite, nepheline syenites (Kishangarh, Rajasthan), charnockite, hyperite, phyroxenites, dunites (Salem), granulites, anorthosite, khondalite (Orissa and Madhya Pradesh), gondite (Madhya Pradesh), kodurite (Vishakhapatnam), marble, pegmatite, quartzite, phyllite etc. many of which are found interbedded in the gneisses and the schists. These rocks also show evidences of progressive metamorphism, i.e., slate metamorphosed to phyllite, phyllite to schist and schist to gneiss.

Distribution

The Archaean rocks are distributed over two- third parts of the Peninsular India extending from Kanniyakumari in the south to Rajasthan in the north. They occupy about 1.87 lakh square kilometres of area in the states of Karnataka, Tamil Nadu, Andhra Pradesh, Orissa, Jharkhand (Chota Nagpur) and Rajasthan (south-eastern). They also form the Central Ranges of the Himalayas from Kashmir to Myanmar (Martaban system and Mogok gneisses of Myanmar).

The Peninsular Gneiss-it is also known as the Bengal gneiss because of its first study in Medinipur district of West Bengal. Owing to its peculiar weath­ering giving rise to ellipsoidal or dome-shaped masses it is called the dome-gneiss. It is a highly foliated rock varying in composition from granite to granodiorite or even amphibolites with abundant ac­cessories such as epidotic, apatite, garnet, sphene,

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