The city was found in layers which indicate that it was rebuilt many times due to the flooding of the Indus River.
- Occupations are detected by traces of ancient materials found in layers, which differ from one another in colour, texture and the artefacts that are found in them.
- Abandonment or desertions, called as “sterile layers”, can be identified by the absence of such traces.
- Generally, the lowest layers are the oldest and the highest are the most recent. The study of these layers is called stratigraphy. Artefacts found in layers can be assigned to specific cultural periods and can thus provide the cultural sequence for a site.
Seals were discovered at Harappa by archaeologists in the early decades of the twentieth century, in layers that were much older than Early Historic levels. It was only then its significance was realised.