The Indus Valley Civilization was one of the world's first great urban civilizations. It flourished in the vast river plains and adjacent regions in what are now Pakistan and western India. The earliest cities became integrated into an extensive urban culture around 4,600 years ago and continued to dominate the region for at least 700 years, from 2600 to 1900 BC. It was only in the 1920's that the buried cities and villages of the Indus valley were recognised by archaeologists as representing an undiscovered civilization. Large cities and smaller towns grew up along the major trade routes as administrative and ritual centres. During the full urban phase of this civilization, there is evidence for trade contact with the surrounding cultures in the Arabian Gulf, West and Central Asia and peninsular India. Urban Character of the Indus Valley Civilization Around 2600 BC the various regional cultures were united in what is called the Indus Valley Civilization. It is also commonly referred to as the Harappan culture after the town of Harappa where it was first discovered. This civilization was organized around cities and towns that were located at major crossroads and in rich agricultural regions. The ruling communities of these cities developed a distinctive form of writing. They appear to have controlled a vast geographical area, some 650, 000 square kilometres. This area is twice as large as that controlled by Mesopotamian or Egyptian cultures at this same time in history. Hundreds of Harappan settlements have been discovered, and archaeologists have been able to excavate different types of sites in each of the major regions. The earliest excavations focused on large cities located along the Indus river and its tributaries; Mohenjo-daro on the Indus) (Sindh, southern Pakistan) and Harappa) on the Ravi River (Punjab, northern Pakistan). Several smaller towns, rural villages, mining, trading and coastal settlements have also been excavated in both Pakistan and western India.