What happen during the c. 1500 BCE?

By about 1500 bce an important change began to occur in the northern half of the Indian subcontinent. The Indus civilization had declined by about 2000 bce (or perhaps as late as 1750 bce), and the stage was being set for a second and more lasting urbanization in the Ganges valley. The new areas of occupation were contiguous with and sometimes overlapping the core of the Harappan area. There was continuity of occupation in the Punjab and Gujarat, and a new thrust toward urbanization came from the migration of peoples from the Punjab into the Ganges valley.

Taken from : http://www.britannica.com/EBchecked/topic/285248/India/46841/From-c-1500-to-c-500-bce

Pictures taken from : http://www.harappa.com/har/har0.html