Gujarat is a land with multiple landforms. It has the mainland Gujarat with flat alluvial plain with a river system that originates in the higher eastern reaches of the state. Then the Saurashtra part of the state is dominated by basalt domes and ridges and Kachh region has the great Rann, the salt pans. Since times immemorial these landforms have been experiencing the nature's ire. The science of geology is such that it gives a third eye to the person well versed with the subject and he can probe the landforms and get answers to several how and whys. Through the following lines I am going to unravel some of the past mysteries of Gujarat as seen by L.S. Chamyal, D.M Maurya and Rachna Raj of Department of geology, M.S. University of Baroda, Vadodara, India. They worked extensively in the state and collected evidences about the past environmental ups and downs of Gujarat as depicted by the landforms and river systems of the state.
Gujarat is an area with a checkered tectonic history. It is an area where in the past the sea has often engulfed the coasts and has retreated again. These actions of nature are preserved in the river systems. Rivers are extremely sensitive to tectonic and sea level changes. Fluvial systems (river systems) of semi arid and arid areas of Gujarat show a strong structural control on he drainage architecture and the landscape of the area, which has evolved due to neotectonism and paleoenvironmental changes, say Chamyal and his colleagues in their paper published in the journal, Quaternary International.
The part of the earth between the Tropic of Cancer on the north of Equator and Tropic of Capricorn, south of Equator is known as the Tropics. The seasons in the Tropics are dominated by the movement of Tropical Rain belt, which oscillates between the northern and the southern tropics over the year, thus causing the dry and the wet seasons in turn, says the Wikipedia. The Tropic of Cancer passes through the state of Gujarat. With Thar Desert bordering the northern part of the state and the Arabian Sea one west, the climate of Gujarat is much influenced by these factors. There is so much variation in the climate in the state due to uneven distribution of rainfall that it is presently categorized in to sub-humid, semi-arid and extremely arid zones.
The climate affects the landforms. In addition the landforms are also affected by the tectonic disturbances. Indian plate as it began colliding with the Tibetan Plate some 65 million years ago (m a), the western continental margin of the plate began to fragment. Consequently three rift valleys were formed at Kachh, Cambay and Narmada. These valleys were connected with the sea till about 1.75 m a as confirmed by the presence of marine sediments in these basins at depths. Once the basins were filled up there were adjustments and rivers began to flow through newly formed plains. Thus the sediments at higher levels are of younger age and have been deposited by the rivers.
The Saurashtra peninsula made of basalts, on the other hand was a tectonic as well as geomorphic high. If Rajkot is taken as centre the streams from there flow out in all directions in Saurashtra. Saurashtra was a high land as the Kachh and Cambay rift valleys were formed. And it remains a highland even today. Thus, on the fringes of Saurashtra even today sediments older than 1.75 m a are found. There are not many studies available on how much was the ground movement and relative sea level change in the past 1.75 million years. It seems during an interglacial period some 125 thousand years ago (ka), the sea level was much higher. As a result it extended inwards in mainland Gujarat as far as Mahudi in the Sabarmati valley, Bhadarwa in the Mahi valley and Kotia in the Narmada valley. This was confirmed by the Infrared stimulated Luminescence studies of the clay rocks found along the onshore areas of the above rivers. It was found that the sea level was seven metres higher than the present day and the retreating sea left behind a thick succession of sediments which are now present as vertical cliffs along various river valleys. These sediments have layers of gravel in between. It seems they were deposited in semi-arid to sub-humid climatic cycles and in between in some rivers like Mahi there were floods which brought the heavier sediments like gravels. Further studies of these sediments showed that they were equivalent in age to Marine Isotopic stages 5 and 3, which is approximately 125 ka to 110 ka.
The wind-born sediments main land Gujarat were dated and it suggests that arid phase set in as early as 22 ka before present (BP). However, a further refinement in dates shows that sand movement was at peak some 14 ka BP. This was the period post Last Glacial Maxima, i.e. 18 ka years BP. At the same time the sea had receded by about 120 m in the Gulf of Cambay and Rann of Kachh. In other words these areas were part of the main land at that time. Gujarat mainland remained arid till about 10 ka. It appears that between 10 ka BP to 2 ka BP there was a humid phase in the mainland of Gujarat. This is indicated by the sediments on the Narmada flood plain. They are of the type of sediments found in the sheet floods. Around 6 ka the sea reached its maximum and encroached upon the mainland through the estuaries and submerged vast areas of Ranns of Kachh, Okha and Nal region says Chamyal. Radiometric dates of sediments from Mahi estuary reveal that the sea remained at the same level till as late as 2 ka BP.
Since times immemorial rivers have fascinated man. Most of the best habitats are along the rivers. The craving for converting the flood pathways for human use continues. Gujarat is no exception. The rivers on the other hand have their own ways. They often confuse even the nature. For example regional slope of the area drained by Sabarmati, Mahi, and Vishwamitri and Orsang rivers is southwest. These rivers on the other hand have apparently captured E'W to ENE'WSW courses of their tributaries. Thus Sabarmati has captured the course of Vatrak, the Mahi has captured the course of Min, Mesri and Goma rivers, the Vishwamitri has captured the course of E'W flowing smaller rivers and the Orsang has captured the E'W flowing Heran and Unch rivers. Rivers often change their courses and capture the channels of others. Often this happens when the subsurface is in turmoil and the drainage is realigned. The drainage system of Gujarat is influenced by the tectonic activity that started in the later part of the last 1.75 million year history of the earth of this region.
The geological, tectonic and climate history of Gujarat needs a thorough detailed study to interpret the possible future changes in the courses of the rivers and also in the behavior of the Rann. It is well known that barely 2000 years BP the Rann of Kachh was not shallow; it was part of the sea.
Gujarat is a progressive state. Despite upheavals in the past it has emerged a winner and has progressed well. A detailed knowledge about the land the coast will surely help the planner to develop the state in harmony with nature.