[CliffordK]

The survivors obtain it from the millions and millions of metal coins left lying around in the debris of the old, now extinct civilisation.  The old coins - the nickels, dimes and quarters, get "mined". Then hammered and beaten into new arrow-heads.

It's a long time since I read "Earth Abides", and my memory of it has faded .  However I do seem to remember that one of the characters in the book - was it "Ish", raises objections to this method of "mining".  On the grounds that it's not the true creation of new artefacts from raw materials, but more akin to mere "scavenging".

I think Ish gets confuted, but can't remember how.

The "value" of antiquities changes over time.  And, a sustenance society would likely put very low value on them. 

Many of our greatest ancient treasures were mined in one form or another including the metals being stripped from the Roman Pantheon, and limestone taken from Egyptian pyramids and marble removed from other antiquities.

Even today we mine coal which we know is full of fossils, as well as many fossil stones without thought of the life that brought it into existence.

With conservation of matter, everything we have ever dug up out of the earth still exists somewhere in one form or another.  Toxicity aside, our landfills could be full of precious metals for future generations.

Over the next few million years, new mineral veins will be formed or exposed, and much of earth will be reset.

[Bored chemist]

A lot of cars would crash quite quickly. The planes that were in the air at the time would take rather longer to fall out of the sky.
Some ships might sail on for days or weeks before they hit something, or ran out of fuel.

[Bored chemist]

With conservation of matter, everything we have ever dug up out of the earth still exists somewhere in one form or another. 

But with the exception of gold- which we dig up in many places, but then bury again at the national banks, we dig up concentrated deposits of minerals, and then distribute them all over the place.

It's going to be a challenge to find shallow coal for a long time.

[CliffordK]

Some ships might sail on for days or weeks before they hit something, or ran out of fuel.

The SS Baychimo was a Ghost Ship sailing without crew for nearly 40 years.

Of course that is just a drop in time.  However, it is possible some of the more armored military ships could sail without crew for quite some time, especially if they got caught in a current that kept them in the middle of one of the oceans.

[ukmicky]

The only immoral acts in the universe are committed by humans. Everything else that happens is just physics and chemistry.

But isn’t our morals and how we act ultimately also down to nothing more than chemistry and physics