Within the subcontinent there are many geographical variations but the major ones are best divided into three parts; Southern, Western and Eastern.
Southern:
Starting from the south, from the extreme southern tip of India at Kanyakumari a mountain range called Sayhyadri begins. It is about 3000 to 4000 feet high and goes in a south to north direction very close to and parallel to the Arabian Sea. Its west face is steep and receives heavy rain; Goa and Mumbai receive about 100 inches of rain per year. Its east face is gentle and receives much smaller amounts of rain, Pune is about 100 miles east of Mumbai (but on the eastern slope) and receives about 30 inches of rain per year.
North of Mumbai about 150 miles the mountains end where the Tapti River separates it from the Vindhya Mountains which run east to west in direction. The Vindhya Mountains start about 50 miles east of the Arabian Sea and extends east more then 1000 miles (just about 100 miles short of The Bay of Bengal). It reaches up to about 4000 feet high, but receives a good amount of rain and has thick tropical forests. (In the olden days the forest was denser then it is now.) The forest was also home to wild animals, snakes, scorpions and the dreaded cobras. It was also full of insects and could produce various diseases. The forest was also home to some tribal groups; some were cannibals and the Aryans called them Rakashasa. (We saw an Indonesian puppet show at Symphony Space in New York which was based on Bhima's victory over some of these so called Rakashasa). These factors helped prevent the northern incursion for a long time.
As a result, the area south of the Vindhya and bounded by the ocean was considered South India and remained largely separated until about 2500 years ago. It developed independently and its people (Dravidians) developed their own languages. Most of the major rivers in the south start from the eastern face of Sahyadri or the southern face of the Vindhya, flowing east and emptying into the Bay of Bengal. The only two exceptions are the Tapti and Narmada Rivers which originate from the southern face of the Vindhya and flow west to the Arabian Sea.
Although the Aryans did not go South for quite a long time, some of them went by sea to the island of Sri Lanka and settle there. Thus the majority of Sri Lankans are of Aryan descent. They are predominantly Buddhist and their language, Sinhalia is of Indo-Aryan origin and related to Sanskrit. Dravid Tamilians, predominantly Hindu, from South India arrived later and settled in the northern part of the Island. The conflict between these two groups has been quite violent over the years and continues to this day.
Western:
North of the Vindhya Mountains and almost in line with the Sahyadri Mountains lay the Aravalli Mountains. They also run north to south, but they receive much less rain than their southern counterparts. As you go north from Mumbai the amount of rain decreases. Surat receives about 60 inches of rain per year, further north Bharuch and Baroda receives about 40 inches and Ahmedabad only 25 inches. Further north, Rajasthan, Sindh and Baluchistan (in Pakistan) receive only a few inches per year. In fact, the whole Western area is arid or semi-arid and also has a large area that is actually desert, Tharparkar also known as the Great Indian desert. The Aravalli Mountains continue north almost to Delhi. Both Udaipur and Jaipur are situated in Aravalli. Further north of Delhi the land is flat for about 100 miles but sits about 1000 feet above sea level and is a water shed between the west and east. Further north are the Siwalic Mountains, which are really foot hills to the Himalayas.
Though arid, the Western region had seven big rivers originating from the Himalayas and fed by snow and glaciers. The valleys of these rivers and the surrounding areas were the birthplace of the ancient Indus Valley civilizations approximately 5000 years ago. This civilization was destroyed mostly due to natural disasters almost 3900 years ago. This region is prone to drought and on the opposite end, sometimes severe flooding due to the sudden melting of snow. Additionally, this area is prone to large earth quakes. (The recent Gujarat earthquake in 2000 and in Pakistan and Kashmir in 2004 are reminders of this.) Any one or a combination of these natural disasters could have brought an end to this civilization.
India and the world got two permanent gifts from this civilization. They grew cotton, learned to spin and weave the cotton and gave birth to textile industry. The people who survived would teach this art to the next great civilization called the Aryan Civilization. Many historians and archaeologists think the Indus people were also the ones who started the practice of yoga and passed it along to the Aryans who then developed it further.
The Aryan Civilization also was settled in the same area about 3700 to 4000 years ago, but was limited to the valleys of the seven rivers and called their area Saptsindhu (Sapt in Sanskrit means seven and Sindhu means river), the land of seven rivers. The Aryans were originally nomads and survived by herding and hunting before settling in the subcontinent. The four Vedas and related literature was produced during this time.
The Aryans considered themselves to be superior to the indigenous people and the survivors of the older civilizations. This belief was held despite the fact that they learned farming and cloth making from these very people. They created a formal class society where their priests (Brahmins) were the superior class. Next in line was the warrior class (Satriyas). The rest of the Aryans who took up farming and crafts made up the third class (Vaishyas). The indigenous and survivor's of older civilizations were the lowest class (Sudras) and were servants, manual workers and laborers.
Some of the indigenous people resisted and went out and lived deep in the forests and in mountain hideouts. Many of them, called Adivasis (aboriginal), still live like this even today in the 21st century where they continue to live in their own tribal ways. Some historians think the Dravidian People of South India are the survivor's of Indus Valley civilization who did not accept the Aryans and their notions of superiority who migrated south along the west cost of India.
Eastern:
The area between the Himalayas and Vindhyas and east of Aravallis Mountains form the Eastern part. It receives greater rainfall then the Western Region and also most of its rivers originate from the Himalayas and ultimately join the Ganga River. The rivers from the northern slope of the Vindhyas also flow north and ultimately meet the Ganga. Rivers from the eastern slopes of Aravalli do the same. Thus the Ganga carries water from almost the whole area out to the sea (The Bay of Bengal).
It is here that the Aryans from the west will migrate around 3200 years ago. The area between Delhi and present day Calcutta will be the center of the Indian civilization for the next 2000 years. The Vedic religion will be transformed into Hinduism, and then Jainism and later on Buddhism will be born from it as well. The area will also give rise to big states and empire's. The capital of pan Indian dynasties will move to different places but will always be in this area. It is also in this area that most Indian myths and legends will develop. All of the places mentioned in the Mahabharata and most of the places in the Ramayana are located in this area. In Jainism all the births of the thirthankars, their diksha, kevalnan and nirvana places are all located in this region. The only exceptions are the first thirthankar’s nirvana in Kailash and twenty-second’s in Girnar, Gujarat. Old literature only sometimes mentions the West and rarely the South.
Places outside the subcontinent were called Anarya Lands (non-Aryan lands) and not worth considering. It was only after Mahavir and Buddha that things began to change. However, it was after Alexander the Great’s invasion and the founding of the Maura Empire that rapid changes would come and the subcontinent would become more like one cultural unit.