A new species with technological potential is starting to multiply and subjugate the Earth. What resources can it harvest from human (our) cities which were destroyed 30M years ago (humans are gone now) ?
Assuming some of the ruined cities still remain close to the surface and were not buried miles underneath by tectonic activity.
Split from the original question: Post-apocalypse: large mammals erased, can homo sapiens 2.0 build civilisation?
Nothing. 30 millions years ago Antarctica developed its ice cap. The Alps STARTED to rise in Europe. The place now known as South America detached from Antarctica and started drifting toward North America.
The Great Pyramid of Giza was built around 4500 years ago. 30 million years ago that part of Africa didn't exist. To give extra perspective. The WHOLE region known as Italy and Greece wasn't there. There was water.
Glass needs around 4000 years to decay. You could make a glass bottle and wait for it completely decay and then make a new one seven thousand five hundred times. 7500 TIMES.
During that 4000 years there would need to be some kind of change that would stop (because slowing down would do nothing) the decay only then would it survive.
The current answers are assuming you mean from 2018 human cities. Since it will probably take some time for our ultimate demise, some technological advances could take place that would allow a species 30 million years later to detect human presence.
It is lucky that a resourceful and vain group of scientists thought of this notion in 21XX. They developed the technology for nanobots!
Each nanobot, as part of its self replicating "DNA", contains the information from the entirety of human written works. The nanobots, while ultimately being of the 'gray goo' type, reproduce very slowly. So they themselves have been migrated by geological events. They are also fragile, so extreme heat, cold, pressure render them unable to reproduce. They have thrived though in places containing unique compounds and higher than normal carbon content (i.e. cities and garbage dumps!). Of course 30 million years is a very long time, so the nanobots also reuse the corpses of non-functioning nanobots. What is left 30 million years later, after millennia of nanobot reproduction and mutation, is areas of the Earth containing pockets of nanobots, each slowly reproducing and containing encoded information from a species long gone.
ADDED BONUS PLOT
The nanobots are quite small, and basically indiscernible from "oddly colored dirt". The information encoded in them is basically irretrievable by all but the most sophisticated of beings. The nanobots, while small, do have primitive locomotion (so they can cluster near high resource locations). They find carbon-based life forms especially delicious, particularly the large spongy masses known as "brains". They are slow to reproduce (decades), so an organism may live its entire normal life with a nanobot embedded, and not notice too much.
There is an unusual effect when a nanobot or a few hundred, invade a brain. The host organism's neural activity is altered by the molecular structure of the nanobot (which contains our encoded information). It manifests itself first as hallucinations, or wild imaginative thoughts. After much training, and the nanobots thoroughly embed themselves within the host, the host is able to enter a trance-like state where vast new worlds of information are available. This leads to the host species having cultural or technological advances that leads to ....
The oldest cities we have on Earth date back to some thousands year ago. Way too young to give a reliable metric on what could remain after 30 million years. But we can give it a shot based on our current knowledge on how various (broad) classes of materials would behave to ageing:
OK, let us start with: after 30 million years, what geological activities did not to the cities, weather, erosion and vegetation did. Even plastic materials are gone, rubber, stainless steel, glass...everything. You'll be lucky to find the pattern of what used to be the cities well hidden inside the greenery.
The polar settlements have been devoured by ice. The mountain communities have long been destroyed by avalanches that inflicted the final blows to the lack of maintenance.
So, perhaps they can find some leftover bricks from the Great Pyramids, they could find some rubble from the dams and China's Great Wall, but without knowing anything about what those were before, thus unvoluntarily deleting what was left of humans' mightiest efforts.
Another useful find, if they wander near a former metropolitan area, could be the leftover from railroad tracks -deformed, crushed tracks without any trace of wood left of course, but they could strike the explorers as weird surface metal veins.
30 Million years means:
Multiple ice ages in temperate latitudes, reforming the ground over and over. Most cities in Europe and Russia and northern North America scraped clean by mile-high rivers of ice over and over. No traces left at all at the site, and curious jumbles of oxidized rich ores at the edges and drop-areas.
Cities along slow rivers buried by sediment and then fossilized. Cities along fast rivers long eroded away, nothing but air remaining.
While your new species might have trouble harvesting anything from the previous human civilization, see all the other answers for details, they may still find out about humanity in a limited way.
An article from Space.com in 2011 put the survivability of the Apollo missions equipment within the time range you're talking about.
"They won't be there forever," Mark Robinson, an Arizona State University scientist and the principal investigator of LRO's camera, said in a news briefing today. "The moon is constantly bombarded with micrometeorites. These are very, very small particles that impact at very high velocities."
"In human terms, it may seem like forever, but in geologic terms, probably there will be no traces of the Apollo exploration in, let's say, ten to a hundred million years," Robinson said.
Very few items would retain their shape and characteristics over that extreme amount of time, due to the scientific reasons proposed in many of these other answers - however, fossilized remains of the preexisting humans can be found, leading our future archaeologists to wonder if they have something to do with the millions of intelligently shaped and carved gemstones that have been found in every corner of the globe.
Surely, this primitive ancient civilization could not have had the technology to form these brilliant gemstones, and it is evidence of ancient alien intervention on our planet. Diamonds, rubies, or emeralds, all shaped and formed, how curious - they must have had a significant role in ancient society.
Over 90 % of naturally occurring uranium is U238, but U235 is used in nuclear reactors because this is the isotope that is fissile.
Future earthlings would find large unexplainable deposits of U235 around the sites of old nuclear reactors.