2 Methodology S.3 Altimetric Analysis 5
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8^ CHAPTER V-: EROSION - SURFACE 5.1 Introduction S»2 Methodology S.3 Altimetric Analysis 5»4 Profile Analysis 5.5 Salient Features of Erosion Surfaces 5*6 Conclusion Appendix ***#*»#*** 85 5.1) INTRODUCTION : The general term "erosion surface " is employed to describe the geographically plain surface which is the end product of complete or incomplete cycle of erosion(Gibert, 1877)* It is synonymous with the term " planation surface "• Many scholars have described the planation surface in differ ent-ways since past as "Peneplain" (Davis 1906), "Pediplain" (Penckl953), "Panplain" (Johnson 1936), ,etchplain"(Wayland 1934 and 'stripped etchsurface*(Budel 1957). Extremely low relief over a flat surface is a salient feature of the planation surface* Erosion surface is an important phenomenon in the evolution of landforms, since it helps to understand geomor- phic processes acting on the land surface in the past and to interpret the present landscape wfcich is essential to know the geomorphic history of that unit area* Thus it leads to comprehend the denudation chronology* The present topographic expression of the Panzara basin has been caused by the continual interplay between geo morphic processes and the tectonic activities on the initial surface. They produce some micro-relief features* They may be either aggradational or degradational surfaces* This chapter aims at detecting different erosion surfaces at various altitudes and leads to comprehend the pily- cyclic nature of landforms in the Panzara basin* 86 5.2) METHODOLOGY : Methodology of the study is based on altimetric ana lysis (Bauling 1926)* Amongst the various altimetric techniques, b the simple frequency distribution of spot heights and the frequency distribution of highest and lowest points in superim posed grids have been adopted. Besides these, a series of super imposed profiles and longitudinal profiles are used for identi fication of knick points and erosional surfaces. One inch topo graphical maps of Survey of India have been uded for investi gation. The confirmation of erosion surfaces has been made in subsequent field check. The existence of mature soil over the surfaces Chambalbedi mal, Salia mal and Pimpal mal at 1500-1750 feet. Occurrence of calcareous deposits over Galana Fort at an altitude of 2250-2500 feet. Existence of residual hill at Bhamer over Chambalbedi mal, occurrence of numerous residual hills over Kondaibari surface are unmistakable evidences of erosion surfaces. 5«3) ALTIMETRIC ANALYSIS : It is a remarkable fact that the highest parts of the present topography will be residual remnants of the former surface • Remnants of former erosion surface can be recognised with the help of altimetric frequency analysis. Araingst the various altimtric techniques, two methods have been adopted in the present study, viz. (i) Simple frequency distributin of spot heights with or without summit frequencies. (ii) Frequency of the highest and lowest points in the super- 87 "I UJ li. oI I I LD o o < (J) < UJ en < f- o < •—I X LU o N X t—4 UJ < o>- x CL i N o UE a. LO LU cr U- Ol 5 UJ < 3aniiiiv 8 CUMULATIVE FREQUENCY CURVES FIG. 2 feel SPOT HEIGHTS 5000 4000- 3000- ^ 2000- en * 1000- x: 0 6 20 40 6b sb 160% cumulative frequency feet MINIMUM HEIGHTS 400C-. 3000- £ 2000H £ 1000-1 0 4 feet ° 20 A0 60 a0 100 /. 5 000-, cumulative frequjence y 5000 MAXIMUM HEIGHTS 4000- 3000- - 20OO- * 1000- 0 20 4'o 6b a'o 160°/. cumulative frequency 8 b imposed square grid from one inch maps of Panzara basin Survey of India, The same techniques are widely used by Indian Geomor- phologists(Desai 1956, Rajkumar Rai 1979, Singh 1980, Mukhopa- tihyay 1981, Nageswar Prasad and Kodali 1984), (A) Simple Frequency Distribution of Spot Heights,: Table No.l shows spot heights frequencies in the height groups of 250 feet class interval. It reveals that the highest concentration of frequencies is observed in between 1500-1750 feet. Lowest frequencies are observed below 500 feet and in between 3750-4000 feet. There is a very slight increase of fre quency with a trace number above 500 feet till 1250 feet. In between 1250-1500 feet, the frequencies have decreases and then abruptly increases in the altitudinal range of 1500-1750 feet. Above 1750 feet altitude, frequencies have again decreased in number. Moreover, the histogram also reveals the same which appears like a bimodal one. For getting a better perspective of the distribution of frequencies, histograms with different altitudinal class intervals have been plotted on the same graph(Fig.l) which reveals the following points* (A.l) The histogram A (Fig.l) shows that the maximum frequen cies are concentrated in between 1000-2000 feet altitude. It suggests a single modal distribution. Moreover, more than 96% of the total frequencies lie below 3000 feet. Above 3000 feet, frequencies are abruptly decreasing which account for only 3.27 % of the total. (A.2) A very low percentage of frequencies has been encountered below 500 feet and above 3000 feet(Fig.l.B) Nonetheless, one may have considered that the surface of the Panzara basin is Su mostly confined to 500-3000 feet. The frequencies are also very low above 3000 feet* The area above 3000 feet mostly comprises of summits on the southern water divide of the Panzara basin. They axe eastern off-shoots of Sahyadri or locally they are faknown as Galana Hills, since the surface above 3000 feet is named as "Galana erosion surface"• (A.3) The histogram B(Fig.l) also appears like bimodal one* The two modal classes, 500-1000 feet and 1500-2000 feet, sftow a higher concentration of frequencies. The mid values of these two classes are 750 feet and 1750 feet,respectively. In the previous chapter(Chapter VI), knick points in a longitudinal profile have been identified at 750 feet and 1750 feet. It is the clear indication of erosion surfaces at altitudes of 750 and 1750 feet. Besides the histogram, cumulative frequency curves have also been drawn, which show marked inflexions(Fig.2). These inflexions points suggest different erosion surfaces. Fig.2 shows marked inflexions in cumulative frequency curves at 1250, 1750, 2250 and 3000 feet altitude. (B ) THE FREQUENCIES OF ..HIGHEST AND LOWEST .POINTS. M SUPERIMPOSED GRIDS : Bauling's(1935) device is used t'o pervade some deficien cies in the spot heights. In his device, Bauling had used a square grid in which frequency distribution of the highest and lowest points is counted and plotted on graph, Bauling(1948) believed that the patterns of the highest points would define an upper enveloping surface which becomes closer to the real surface as the local relief becomes more gentle. He contended UJ - UJ • i 1 o o o _J If) o o < CM in o > cc ui 1 • H | z: J .—I UJ -i- >-°1l -z. UJ 10 =3 •—•! o CO UJ >- a _J ^ < n :<Z sx 1 U4 X B-PC 2l *- z. LU 1—ll ZE » t i H; -J , <r 1 o UJ o UJ o u_ <r qan±inv dk o I—H Lu o :z LU CO o 1—4 LU CO >- —J < X < o I—1 LU X 0 1—I ~F a: 3 1— 2 UJ 1—1 "^ < >—1 2 I— _J < aanmiv that the concave curves in the altimetric plot indicate the mature stage*,where the region is affected by a single cycle of erosion. The inflexions or segments produced in a curve suggest the different cycles or polycyclic nature of landforms. With reference to Bauling's device, one inch Survey of India map of the entire Panzara basin can be divided into square grids(0.5745 x 0*5745 inch on map) each of which covers 0.33 sq.mile of area. The highest and lowest points in each square grid have been estimated and tabulated in Table Nos.2 and 3 at 250 feet class intervals of altitude. Moreover, the histograms have also been plotted on the information obtained from Table Nos.2 and 3 (Figs. 3 and 4). It is obvious from Figs.3 and 4 that the maximum frequ ency belongs to 1000-1250 feet, which accounts for 17.35 per cent in the case of the highest elevated points in superimposed grids. However, maximum frequency of lowest point in superim posed grid is observed in the highest class 750-1000 feet. Owing to this fact one may be led to consider that the real surface has been observed belonging to 750-1000 feet, referring to the highest frequency which accounts for more than 21 %• (Table No.3, Fig,3). The upper enveloping surface to this real erosional surface lies in height group 1000-1250 feet which can be predicted from the observation of highest frequency of lowest point in superimposed grids in class 1000-1250 feet. (Table No*2, Fig.3). It is believed that the maximum elevation of the dykes range is 1250 feet which is explicit from the S.O.I, toposheets of Panzara basin. A very gentle scarp like surface is observed in between 1250-1500 feet in subsequent field check. It is also marked by the occurrence of low frequencies in the group 1250-1500 feet in both spot heights and frequency of highest and lowest points in superimposed grids. Following this class, frequencies have suddenly increased in class 1800-1750 feet* This tendency is observed in both spot heights and grid- wise frequency of highest and lowest points,(Table Nos.1,2,3 and Figs.l A,B,C)f which can be predominently marked by an eventful erosion surface. It covers an extensive plateau surface with mature soil cover. This surface covers an extensive table land bordering by steep edges like Chambarbedi Malt Pimpal Mal,Salia Mai, Trishul Mai, etc.