Long term evidence of civilization.

[Uncwilly]

Large dams, airport runways (many are single pours that are thicker than a person is tall), subway tunnels, and while it may be a jumble, downtown Manhattan (or megacity of your choice) should leave evidence for a good long time. They may not stay in good shape, but they should remain.

[xilman]

Originally Posted by Uncwilly

Large dams, airport runways (many are single pours that are thicker than a person is tall), subway tunnels, and while it may be a jumble, downtown Manhattan (or megacity of your choice) should leave evidence for a good long time. They may not stay in good shape, but they should remain.

All of which are extremely prone to weathering unless they are located in gently sinking mud such as are to be found at New Orleans and Amsterdam. Manhattan is not a good example. One half-way decent glaciation event and it's ground into gravel and then dumped into the Atlantic. I don't rate its survival very highly for even 100K years, let alone 100M.

Oh sure, some your examples may hang around relatively recognizably for the near future, the next 100K to 5M years say, but not for 100M years.

Concrete crumbles and dissolves very rapidly when buried a kilometre in very hot water. Quite a bit of Roman concrete, 2K years old, still exists but very far from most of it and that is still on the surface.

[xilman]

Originally Posted by chris2be8

Remains of roads, buildings etc buried in sediment would be a lot easier to find than animal fossils.

Perhaps. Given that roads, buildings, etc are rock it is not so clear to me.

[chris2be8]

There are many different sorts of rock (composition, grain size, orientation of layers etc). A fossil of a brick would still have a rectangular shape which would not occur naturally. And the cement in a wall is made of coarser sand than the clay the bricks are made of. So it could be heavily distorted and still be recognisably artificial to a geologist.

And think how many buildings there are on earth. Some would be bound to leave recognizable traces for a very long time.

Chris

[xilman]

Originally Posted by chris2be8

There are many different sorts of rock (composition, grain size, orientation of layers etc). A fossil of a brick would still have a rectangular shape which would not occur naturally. And the cement in a wall is made of coarser sand than the clay the bricks are made of. So it could be heavily distorted and still be recognisably artificial to a geologist.

And think how many buildings there are on earth. Some would be bound to leave recognizable traces for a very long time.

Chris

After some megayears of squidging (technical term) at high pressure, high temperature, high humidity, essentially all bricks will end up as reconstituted clay and not at all rectangular. That assumes they get buried in the first place. Erosion from plants, water and wind will get them in short order otherwise.

[chris2be8]

I agree that after a few million years there will be very few obvious signs of todays civilisation on the surface of the earth. But stuff that got buried in sediment will survive in large enough quantities that any geological survey of the earth would find some.

And signs of tunnels etc from mining in stable parts of the earth would be rather obvious to anyone trying to mine the same resources.

Chris

[rogue]

Originally Posted by xilman

I don't have the time and knowledge yet to answer your chemical marker questions. However, I think you will find that high altitude orbits are unstable on timescales of a megayear or so due to planetary and (especially) lunar perturbations.

After a few megayears they will not be highly reflective. Proton and dust bombardment will see to that.

Anyway, even something which is highly reflective and in a high orbit is not easy to resolve. Easy exercise for the reader: calculate the angle subtended by a 1m object at a distance of 100,000 km. Then compute the aperture of a diffraction limited telescope working at 500nm wavelength required to produce an image of that object which is 2 pixels across.

In passing, your naked-eye acuity is quite phenomenal. Geostationary satellites tend to be 10th magnitude or fainter. They are at 40,000 km, highly reflective, and a few metres across.

What about the moon and the items left behind by our trips to it? Quick searches in google didn't reveal how much the moon has changed (or not changed) in the past 1e8 years.

[Uncwilly]

Originally Posted by rogue

What about the moon and the items left behind by our trips to it? Quick searches in google didn't reveal how much the moon has changed (or not changed) in the past 1e8 years.

Hmm, I bet the idea of sticking something on the moon as some sort of sentinel would make for a great Sci Fi story.

[rogue]

Originally Posted by Uncwilly

Hmm, I bet the idea of sticking something on the moon as some sort of sentinel would make for a great Sci Fi story.

Something like this: Svalbard Global Seed Vault?

[Uncwilly]

Originally Posted by rogue

Something like this: Svalbard Global Seed Vault?

No, I was thinking something more like the following and its successor..

[Dr Sardonicus]

Originally Posted by xilman

After some megayears of squidging (technical term) at high pressure, high temperature, high humidity, essentially all bricks will end up as reconstituted clay and not at all rectangular. That assumes they get buried in the first place. Erosion from plants, water and wind will get them in short order otherwise.

Bricks exposed to the air also deteriorate due to weathering.

Bricks exposed to both air and groundwater (or salt water) also may not last.

Regarding the degradability of clay objects, this article may be of interest.