Discuss the depositional and climatic conditions during the Siwalik times.

Depositional Environments

The Siwalik sediments were deposited in a variety of environments, primarily controlled by the proximity to the rising Himalayas and the changing base level. These environments can be broadly categorized as follows:

1. Fluvial Systems

Dominantly, the Siwaliks were deposited by large, braided river systems originating from the Himalayas. These rivers carried enormous sediment loads, resulting in thick conglomerate, sandstone, and mudstone sequences. The presence of channel forms, cross-bedding, and imbricated pebbles indicate high-energy flow regimes. Early Siwalik fluvial systems were characterized by coarse-grained sediments, reflecting proximity to the mountain front. As deposition progressed, the sediment became finer-grained, indicating a shift in depositional centers further away from the Himalayas.

2. Lacustrine Environments

Interspersed within the fluvial sequences are lacustrine (lake) deposits, particularly prominent in the middle and upper Siwaliks. These deposits consist of fine-grained sandstones, siltstones, and mudstones, often containing fossiliferous layers. The formation of lakes was likely due to local tectonic depressions and damming by alluvial fans. These lakes provided habitats for diverse aquatic fauna, including fish, turtles, and crocodiles.

3. Aeolian Environments

Evidence of aeolian (wind-blown) activity is found in the upper Siwaliks, particularly in the form of wind-rippled sandstones and loess deposits. This suggests an increase in aridity and the expansion of desert-like conditions during the late Siwalik period. The aeolian sediments are often interbedded with fluvial deposits, indicating a fluctuating climate.

Climatic Conditions

The climate during the Siwalik times underwent significant changes, influenced by the uplift of the Himalayas, the evolution of the Indian monsoon system, and global climatic fluctuations.

1. Early Siwalik (Miocene – ~23 to 16 million years ago)

The early Siwalik period was characterized by a relatively warm and humid climate. The monsoon was less intense than today, but still played a significant role in sediment transport and deposition. The vegetation was predominantly subtropical, with forests covering large areas. Faunal assemblages included proboscideans (ancestors of elephants), rhinos, and various ungulates adapted to forested environments.

2. Middle Siwalik (Miocene-Pliocene – ~16 to 7.25 million years ago)

The middle Siwalik period witnessed a gradual drying trend. The monsoon became more variable, with periods of intense rainfall alternating with prolonged droughts. This led to the expansion of grasslands and the decline of forests. The faunal assemblages shifted towards more open-country species, such as grazing herbivores and their predators. The presence of Hipparion, a three-toed horse, is characteristic of this period.

3. Late Siwalik (Pliocene-Pleistocene – ~7.25 to 2.58 million years ago)

The late Siwalik period was marked by increasing aridity and the intensification of the monsoon. The climate became more seasonal, with distinct wet and dry seasons. The expansion of grasslands continued, and desert-like conditions prevailed in some areas. The faunal assemblages were dominated by large grazing mammals, such as elephants, rhinos, and bovids. The appearance of early hominids in the late Siwaliks suggests a link between climatic change and human evolution.

Siwalik Stage	Depositional Environment	Climatic Conditions	Faunal Characteristics
Early Siwalik	Braided Rivers, Alluvial Fans	Warm, Humid, Less Intense Monsoon	Proboscideans, Rhinos, Forest-dwelling Ungulates
Middle Siwalik	Braided Rivers, Lakes	Drying Trend, Variable Monsoon	Hipparion, Grazing Herbivores
Late Siwalik	Braided Rivers, Aeolian Deposits	Arid, Intense Monsoon, Seasonal Climate	Large Grazing Mammals, Early Hominids