Explanation:

The northern plain of India is a vast region that stretches across several states including Punjab, Haryana, Uttar Pradesh, Bihar, and West Bengal. It is one of the most fertile and agriculturally productive regions in India. The formation of the northern plain is primarily attributed to the deposition of alluvial soil.

Alluvial Soil:
Alluvial soil is formed by the deposition of sediments brought by rivers and streams. These sediments consist of rich organic material, clay, silt, sand, and gravel. Over time, the continuous deposition of these sediments has resulted in the formation of a thick layer of fertile alluvial soil in the northern plain.

Factors Contributing to the Formation:
Several factors have contributed to the formation of alluvial soil in the northern plain:

1. Rivers: The region is crisscrossed by several rivers, including the Ganges, Yamuna, Brahmaputra, and their tributaries. These rivers bring along large amounts of sediment from the Himalayas and deposit them in the plains.

2. Tectonic Activities: The northern plain is located in a seismically active zone. The tectonic activities, such as the uplifting of the Himalayas, have led to the erosion of rocks and the transportation of sediments to the plains.

3. Glacial Activities: The glaciers in the Himalayas also contribute to the formation of alluvial soil. As the glaciers melt, they release sediments and carry them downstream to the plains.

4. Weathering and Erosion: The weathering and erosion of rocks in the Himalayas and the Aravalli range also play a significant role in the formation of alluvial soil. The rocks break down into smaller particles, which are then carried by rivers and deposited in the plains.

Characteristics of Alluvial Soil:
The alluvial soil in the northern plain is known for its fertility and high agricultural productivity. It has a high percentage of clay, silt, and sand, which makes it well-drained and suitable for cultivation. The soil is rich in organic matter and nutrients, making it ideal for the growth of crops.

Conclusion:
In conclusion, the northern plain of India is primarily formed of alluvial soil, which is the result of the deposition of sediments by rivers and streams. The fertile nature of this soil has made the region a major agricultural hub in the country.