Why are mounds sometimes the sites of ancient cities?

Did you know that large mounds sometimes have ancient cities hidden beneath them?

In fact, the Indus Valley Civilization was discovered when an archeologist, R. D. Bannerjee, visited Mohenjodaro, and saw a large mound. He thought there might be something underneath, and started digging. As the mud was gradually removed, an entire city was revealed!


image source: wikimedia commons

When an ancient city began to decline, people left it, and moved elsewhere. After hundreds of years, the abandoned buildings crumbled, and broke down. The wind blew mud and sand over them, and when the rain fell, the mud stuck to the buildings. In this way, the whole city was covered by mud, and as more mud and sand were deposited by the wind, a mound was formed.

Harappa, and the other cities of the Indus Valley Civilization were discovered when such mounds were dug up.