How did it happen that ancient civilizations buried under layers of dirt and newer civilizations?

Answer 1

Cities and towns have a habit of growing in logical geographic locations for various social, economic or military reasons. For instance, near a water or food supply, next to a road, on a hill, at a natural port, and so on.

When a civilisation dies, the importance of the location often does not die with it, and therefore the site continues to be occupied.

It is far easier to demolish only parts of the existing structure (if they are still standing) and to re-use existing foundations when building from new, so newer cities and towns would often be built over the remains of the previous city or town. Over a long period of time, the "ancient" civilisation will become buried under layer upon layer of more modern civilisations.

Occasionally, and Egypt is a prime example of this, the city or town may simply be swallowed up by encroaching desert sand and become buried.

Answer 2

As the world continues to turn, sand is constintly in motion. Over the years things become burried that way. Especially in the eastern part of the world. The world evolves each and every day. Sometimes it takes a drastic weather change to somehow bury the ruines.

Generally, thousand of years of shifting sand, dirt and water are the cause of the burrials.

Answer 3

Also, it was common practice when a city was overrun or otherwise destroyed, the new city was built on top of it because of the strategic location or natural resources of the original city. The proverbial "rising out of the ashes" concept, if you will.