Marshall suggested that Harappan civilization flourished between 3,250 and 2,750 B.C.
Wheeler dated it to 2,500-1,500 B.C.
On the basis of radiocarbon dating method following chronology of the civilization emerges as −
Early Harappan Phase: c. 3,500 – 2,600 B.C.
Mature Harappan Phase: c. 2,600 - 1,900 B.C.
Late Harappan Phase: c. 1,900 – 1,300 B.C.
John Marshall (Director-General of the Archaeological Survey of India from 1902 to 1928) declares that Harappan civilization declined due to environmental degradation. The cutting of forests for agricultural land and timber for fuel and over-exploitation of resources, etc. resulted in the land to become barren and in the silting of rivers.
The environmental degradation, flood, drought, and famine must have become a recurring feature, which finally led to its decline.
Wheeler opined that it was destroyed by the Barabarian Aryans who came to India in about 1,500 B.C.
The archaeological or biological evidence proved that Wheeler's thesis of Aryan was the destroyer of the Harappan civilization was a myth.
The Harappan civilization was spread over a large area. There may be many causes of its decline as −
In the River Saraswati region, most likely, it declined mainly due to shifting of river channels.
Along the River Indus, most likely, it declined largely due to recurring floods.
Rainfall declined in general, which affected the agriculture, the main economic resource.
With the decline in economic conditions all other institutions like trade and commerce, administrative and political structures, Civic amenities, etc. also declined over a period of time.
Archaeological evidence shows that Harappan civilization did not disappear suddenly.
The decline was gradual and slow, which is witnessed over a period of almost 600 years from c. 1,900-1,300 B.C.
Features such as town-planning, grid patterns, drainage system, standard weights, and measures etc. slowly disappeared and a kind of realization takes place with distinctive regional variations.