I have a question about ancient ruins.

[DetlefK]

How come that from some ancient ruins only the groundfloor or the basement are left?

I have heard that there was the practice to recycle old buildings for their bricks and stones. (Up until medieval times Muslims recycled ancient christian churches in the Middle-East to build mosques.)

Is that the reason why some ancient ruins are intact and some are basically gone except for the floor?

[mdelaguna]

1. A few reasons

You are right. Recycling of building materials was common. Also, often multiple building phases are present. To add a new layer it helped to flatten things out to use the old structure as platform fill. Later buildings toward the surface are more subject to natural or cultural processes of destruction so often digging finds better preserved earlier foundations. Or people are more interested in unearthing earlier remains. Why do some preserve? Rapid abandonment of an area helps (no later scavenging or re-use), or an edifice becomes enshrined and maintained in social memory and is preserved. Also walls & roofs are often made of more perishable materials (wood, plaster, mortar, and will collapse over time or were affected by fire. Some really well built buildings hold up better. (I’m an archaeologist).

[PoindexterOglethorpe]

2. Also, things fall down.

It depends somewhat on the materials used in the first place, and whether or not the building or city was completely abandoned and no one ever went there again, but over decades, hundreds, and thousands of years, things crumble and fall down.

The time span for human built things is relatively short, just a few thousand years, but over hundreds of thousands or even millions of years, absolutely nothing would be left. I've recently read something about that, which pointed out that on about a two million year time frame, we would not be able to detect any kind of artificial structure. Which means there could theoretically have been a technological civilization as recently as two million years ago, and we'd have no way of knowing about them. Let alone, say, a technological dinosaur civilization.