The ancient conception of the Himalaya is one of utter immanence, the eternal home of the gods and goddess. It was an object of awe and devotion and not for men to enquire and fathom. However a reflection on the genesis of the shaligram ammonite, the black fossil revered by Hindus as an embodiment of Visnu, leads to a geological past far beyond the age of man. The making of the ammonite fossil is related to the initial emergence of the Himalayan heights from the depths of a sea in the beginning of time.
           
It was only seven decades ago that Alfred Wegener first postulated the theory of continental Drift. The continents as we know them today were said to have broken off from a single land mass some hundreds of millions of years ago and drifted apart riding on a underlying plastic materials. Though discussions persists as to the actual cause and extend of the drift, modern geo-techniques have reaffirmed the movement of continents, or plates within the theory of plate tectonics.

It is now largely accepted that the Himalaya were formed as a result of the collision of two large movement of continental plates, the Indian subcontinent and Eurasia, in a process that began as early as 130 million years ago at the time when reptiles and dinosaurs roamed the earth. Australia, Antarctica, Africa and parts of south America- the Indian subcontinent traveled 4400 kilometers northwards at an estimated rate of 20-25 centimeters per year and began colliding with the northern land mass, called Laurasia, approximately 50-60 million years ago.

Studies of rock layers have been used to reconstruct the origins of the Himalaya, suggesting early periods of alternating subsidence and uplift of the earth’s surface. An extensive sea existed in the region where the Himalaya now rise, stretching right across the southern margin of Eurasia wedged between Laurasia and Gondwanaland. The sea, known as the Tethys, came into being some 250 million years ago during the late palaeozoic era, when the first reptiles appeared, and dried up gradually about 40-50 million years ago, when mammals came into being. Some scientists now think that the tethys was actually a series of seas that repeatedly subsided, were uplifted and drained away with the passing of a number of land fragments set adrift from Gondwana which followed each other on a collision course into the northern asian continent.

During this time, almost all of the now highly elevated areas between India and central asia were invaded by the tethys sea. Sinking and widening of the earth surface commenced around 200 million years ago. It is believed that subsequent rising of the sub-surface brought the sea to a shallow level towards the lower cretaceous (110-135 million) years ago. By 40-65 million years ago, the bottom had risen so much as to cause the water to spill over and flood much of the surrounding land. This deluge was followed by the ultimate dissolution of the Himalayan sea, as evident by the fact that all later rock types found within the Himalaya (except those in localized basins) were laid down above water.

Thus, the rising of the Tethys sea was primarily due to the build up of marine sediments, accumulating to great thickness of over 4500 meters over a period of some 200 million years, earlier however, during the Jurassic and late cretaceous era (70-80-95 million years) ago, upheavals created some minor submarine ridges and valleys, accompanied by the appearance of volcanoes and subterranean bodies of molten rock. The upheavals were the first spasms of the birth of the Himalaya, which actually took place in a series of stupendous periods of uplift punctuated by intervals of comparative quiescence.

A second upheaval happened in the upper Eocene (38-45 million years ago), raising the primary ridges and basins of the Tethy’s sea into mountain ranges with intervening shallow marshes and large river valleys. It was, however, the intense mountain building epoch of the mid-miocene (seven to 26 million years ago) that created the major structure of the present day Himalaya. This third Himalayan uplift was followed by a dormant period, coinciding with the ice age, when these first Himalayan chains were eroded down to form the Siwalik  hills to the south. Siwalik deposits consist of coarse boulder conglomerates, 1000 to 1500 meters thick.

The fourth sequence in the uplift of the Himalaya occurred about two million years ago when older layers of rock were over thrust into the deposits of the Siwalik. This over-riding is well demarcated by the fault plane called the main boundary thrust. The Pleistocene period, one to two million years ago, a time of much glacial activity when the progenitors of man were stirring. Its impact was felt most in the lower hills of the Himalaya where layers of rock were pushed up as much as 2000 meters. Ever since the first collision of continents, the Himalayan region has been subjected to compression, contortion, elevation and denundation.

The area is still in the process of adjustment: the Indian subcontinent continues to push into Asia at a rate of about two inches per year, as substantiated by the frequency of large slips along major faultline beneath the Himalaya causing periodic earthquakes, as well as by more localized geologic events. In fact, most geologists agree that the Himalaya is still rising, nothing more recent over thrusting in the foothills and a 50 degree tilt of rock layers in the western Himalaya. Kathmandu lake deposits which now dip northwards were uplifted 200 meters over the last 200000 years. The present rate of uplift is difficult to tell as accurate measurements have only been made over the last hundred years.

At the same time, the Himalaya are wearing down, as all young mountains do. The monsoon rains pound at their sides, and constant freezing and thawing cracks the rocks which cause them to shed their outer layers. Although glacial fields are limited, they more than any constant force chisel away at the peaks and carve away the valleys. Season upon season of snow accumulates and is compressed into ice to depths of several hundred meters. The sheer weight shoves the glaciers lowest edge down the mountain, scraping away debris from the sides and bottom. At the top, it scoops out a rounded valley, a cirque, which defines the mountains ridges in bowls and sharp rims. From its terminus, a milky stream runs thick with finely ground sediments which will eventually wash all the way down to the Ganges, returning once submarine deposits back to the sea.

But for the narrow strip of plain along Nepal’s southern border, and temperate valleys spread across its middle, the country is entirely mountainous. The northern part of the country is characterized by towering ice and snow ranges with occasional sparse valleys. This is the Himalayan or mountain region of the country, a part of Nepal conspicuous for its extreme altitude and wild terrain. The highest ranges are crowned by jagged peaks. Ice-scooped basins found at lower elevations indicate the much wider glacial provenance in the past. 

Mountain relief is asymmetrical, with rock strata inclined to the north, leaving steep south faces. The south-tending spurs of the main range are covered with temperate forests lower down and confine steep valleys marked with steep valleys marked with occasional waterfalls. Thunderstorms are frequent and winter frosts limit agriculture. Nevertheless, potatoes are grown to 4000 meters and barley even higher. North of the main range, the prospect is much more desolate with bare mountain slopes and undulating valley bottoms filled with rock debris and sparse vegetation in sheltered corners. This mountain region is a marginal area for human settlement and hence mans influence on the landscape is minimal. Summers are short, winters severe and dry with high snowfall, low temperatures and strong winds. In the northwest of the country a fourth, trans- Himalayan range defines the boundary between Nepal and Tibet. Peaks of 6000 to 7000 meters lie about 35 kilometers north of the main Himalaya, their relief is less rugged, and wind-eroded landforms predominate.

Ere elevated Bhot valleys, broad with open profiles and arid climate – are reminiscent of Tibet, particularly where the Himalayan rain shadow blocks out the monsoon rains. Below the Himalaya, running in a similar west-northwest to east southeast direction 90 kilometers south of the great rise is the Mahabharat range, reaching elevations between 1500 and 2700 meters. These are referred to as the sub- Himalaya. The middle hill region, also called the Pahar, extends between the Mahabharat and the high Himalaya. Its characteristic land-forms are low hills and sinuous ridges, dissected by numerous river valleys. While the smaller valleys make narrow, steep defiles, the larger ones have an easy gradient and a wide open character.

The main north south valleys and their upper tributary extensions make deep indentations in the middle hill topography and these low valleys have numerous old river terraces indicating changing geologic or climatic conditions at the time of alluvial deposits. Landslides and landslips are common and tributary streams overloaded with washed-down materials, unload alluvial cones at their termini. The mild subtropical climate and adequate rainfall have made the midland area a favorable zone for agricultural settlement. Farmers have cleared vast hillsides of trees for cultivation, spoiling the natural landscape. The typical scenery of the middle hill country is flights of terraced fields carved out of steep slopes.

The clement kathmandu valley falls in this belt. The moderate climate permits three harvests a year and small plantings in between. Summer maximums are about 30 degree centigrade and main winter temperatures about 10 degree. Winters are sometimes frosty, but are dry and snowless, while summer monsoons bring substantial rain. Visitors are often surprised to learn that kathmandus latitude about 27 degree 40 north –